# Why do we see a different color when we mix two colors?

The compounds responsible for a color do not change when they are mixed with another material. The same compounds are there after mixing. However, when we mix colors such as blue and yellow we see green.

BUT we are able to tell where a color starts and ends in pictures. For example, if we hold a yellow piece of paper up to the sky, we can tell that the paper is yellow and the sky is blue. We can distinguish where the paper ends. And where the sky starts. We do not see a green outline near the paper.

I believe that how we perceive colors may explain this. But I am not sure how. Is it because how the brain integrates information. Or is it because of how the cones cannot register two signals at the same time. Why do we see green instead of a blue and yellow?

I am not sure if this is entirely correct, its just a scientific guess. But it might actually play some role in this phenomenon. This is actually the physics part part of answer rather than the biology part.

Rayleigh criterion for angular resolution is the minimum angle which two bodies should subtend on an object so that they can be viewed as distinct bodies by that object.

Suppose there are 2 bodies, (right now) with same color of wavelength \$lambda\$. Now, they are viewed from a lens of aperture \$d\$. Thus, from Rayleigh's equation, the minimum angle which these bodies should subtend with each other on the lens is given by:

\$ heta = 1.22 lambda/d\$

Now I'm not sure if this is correct, but if the 2 bodies have different colors, of wavelengths \$lambda_1\$ and \$lambda_2\$ then the equation becomes:

\$ heta = 1.22 imes (lambda_2 - lambda_1)/d\$

\$ heta = 1.22 imes Deltalambda/d\$

Now, putting values in this equation:

\$lambda_2 = 597 nm\$

\$lambda_1 = 492 nm\$

\$d = 5 mm = 5 imes 10^{-3} m\$

Answer comes out as:

\$ heta = underline{1.22 imes 105 imes 10^{-9}}\$
\$hspace{15mm}5 imes 10^{-3}\$

\$ heta = 2.56 imes 10^{-5} rad = 1.4 6 imes 10^{-3circ}\$

So, if these 2 bodies subtend an angle less than \$2.56 imes10^{-5}\$ radian, then they will appear as a single object with green color.

Now, solving it by a common formula:

\$ heta = a/l\$

\$l = a/ heta\$

\$l = underline{hspace{7mm}10^{-2}hspace{7mm}}\$
\$hspace{8mm}2.56 imes 10^{-5}\$

\$l = 390.625 m\$

It means if yould hold two 5 cm x 2 cm papers together (i.e. total 10 cm2), one with blue color and the other with yellow color, then you would see a single paper of green color if you keep these papers \$approx\$391 m away (if my calculations are correct ;).

This, in part, explains that it is not the fault of only our eyes, but of light and physical laws too.

References:

1. Angular resolution

2. Wavelengths of different colors

3. Resolving power of imaging instruments

## Why are red, yellow, and blue the primary colors in painting but computer screens use red, green, and blue?

Red, yellow, and blue are not the main primary colors of painting, and in fact are not very good primary colors for any application.

First of all, you can define any colors you want to be the "primary colors" of your color system, so that other colors are obtained by mixing the primary colors. Although there may be an infinite number of color systems, they are not all equally useful, practical, or effective. For instance, I am free to create a color system where I define light blue, medium blue, and violet as my primary colors. Even though I am free to define my primary colors as such, this color system is not very useful in general because no amount of mixing of these primary colors will produce red, orange, yellow, etc. Therefore, we should make a distinction between a color system and an effective color system. The effectiveness of a color system is best measured as the number of different colors that can be created by mixing the primary colors of the system. This set of colors is called the "color gamut" of the system. A color system with a large gamut is more able to effectively represent a wide variety of images containing different colors.

The most effective color systems are those that closely match the physical workings of the human eye, since it is ultimately the human eye which experiences the color. The human eye contains a curved array of light-sensing cells shaped like little cones and rods. Colored light is detected by the cone cells. The cone cells come in three varieties: red-detecting, green-detecting, and blue-detecting. They are so named because the red cone cells mostly detect red light, the green cone cells mostly detect green light, and the blue cone cells mostly detect blue light. Note that even though a red cone cell predominantly detects the color red, it can also detect a little bit of some other colors. Therefore, even though humans do not have yellow cone cells, we can still see yellow light when it triggers a red cone cell and a green cone cell. In this way, humans have a built-in color decoding mechanism which enables us to experience millions of colors, although we only have vision cells that predominantly see red, green, and blue. It should be obvious at this point that the most effective color systems are ones that closely match the human eye, i.e. color systems that mix red, green, and blue light.

There is a slight complication because there are really two main ways to create a light beam. We can either create the light directly using light sources or we can reflect white light off of a material that absorbs certain colors. A system that creates light directly is called an "additive" color system since the colors from the different light sources add together to give the final beam of light. Examples of additive color systems are computer screens. Each image pixel of a computer screen is just a small collection of light sources emitting different colors. If you display an image of a pumpkin on your computer screen, you have not really turned on any orange-emitting light sources in the screen. Rather, you have turned on tiny red-emitting light sources as well as tiny green-emitting light sources in the screen, and the red and green light add together to make orange.

In contrast to an additive system, color systems that remove colors through absorption are called "subtractive" color systems. They are called this because the final color is achieved by starting with white light (which contains all colors) and then subtracting away certain colors, leaving other colors. Examples of subtractive color systems are paints, pigments, and inks. An orange pumpkin that you see printed in a newspaper is not necessarily created by spraying orange ink on the paper. Rather, yellow ink and magenta ink are sprayed onto the paper. The yellow ink absorbs blue light and a little green and red from the white light beam, while the magenta ink absorbs green light and a little blue and red, leaving only orange to be reflected back.

There are therefore two equally-valid methods for creating color: additive systems and subtractive systems. With this in mind, there are thus two color systems that are most effective (i.e. most able to match the human eye): (1) an additive system that creates red, green, and blue light and, (2) a subtractive system that creates red, green, and blue light.

For an additive system, light is created directly. This means that the primary colors of the most effective additive color system are simply red, green, and blue (RGB). This is why most computer screens, from iPods to televisions, contain a grid of little red-, green-, and blue-emitting light sources.

For a subtractive color system, a certain reflected color is obtained by absorbing the opposite color. Therefore, the primary colors of the most effective subtractive system are the opposites of red, green, and blue, which happen to be cyan, magenta, and yellow (CMY). This is why most printed images contain a grid of little cyan, magenta, and yellow dots of ink. Cyan is the opposite of red and is halfway between green and blue. Magenta is the opposite of green and is halfway between blue and red, and yellow is the opposite of blue and is halfway between red and green.

In summary, the most effective color systems are red-green-blue for additive color systems and cyan-magenta-yellow for subtractive color systems.

So where did the red-yellow-blue color system come from that they teach in elementary school? Typically, students first encounter color concepts when painting in an art class in grade school. Paint is a subtractive color system, and therefore the most effective primary colors for painting are cyan, magenta, and yellow. Note that high-quality paintings typically do not use just three primary colors since more vivid scenes can be achieved using dozens of primary colors. But when teaching art, it's easier to start more simply with just three primary colors. Now, to a little grade-schooler, the words "cyan" and "magenta" don't mean much. Furthermore, to an undiscerning youngster's eye, cyan looks awfully close to blue and magenta looks awfully close to red. Therefore, cyan-magneta-yellow becomes corrupted to blue-red-yellow. Elementary art teachers either ignorantly perpetuate this less effective color model (because that's how they were taught as children), or intentionally perpetuate it (because it's just too hard to teach six-year-old's the difference between cyan and blue). Historical tradition was also a prime driver of the red-yellow-blue color system since it was historically thought to be effective before the details of human vision were understood. Since the red-yellow-blue color system is less effective, it is not really used anywhere these days except in elementary school art.

## What makes things coloured – the physics behind it

It’s hard to image a world without colours simply because they’re all around us. Have you ever wondered, though, where do colours come from? To answer this question, we first have to understand how human colour perception works and how matter physically interacts with light.

### What gives colour

Image: Food Navigator

White light is a mixture of all colours, including those that the human eye can’t see. When we say something has colour, what we actually mean is that light of a particular range of wavelengths is reflected more strongly than the light of other wavelengths. How matter behaves in the presence of light, consequently appearing coloured to us humans, depends on a couple of major factors. First of all — everything is made up of electrons and atoms, but each substance has a different number of atoms and different electron configuration. This way, when light hits matter one or more of the following phenomena happens:

• reflection and scattering. Most objects reflect light, but some are more reflective than others, like metals. This is directly related to the number of free electrons that are able to pass from atom to atom with ease. Instead of absorbing energy from the light, the free electrons vibrate and the light energy is sent out of the material at the same frequency as the original light coming in.
• absorption. When there’s no reflection (the object is opaque), then the incoming light source frequency is the same as, or very close to, the vibration frequency of the electrons in the given material. The electrons thus absorb most of the incoming energy, with little or no reflection.
• transmission. If the incoming light energy is much lower or much higher than that required for the electrons comprising an object to vibrate, then the light source will pass through the material unchanged. This way matter will look transparent to the human eye, such as in the case of glass.
• refraction. If the energy of the incoming light is the same as the vibration frequency of the electrons in the material, light is able to go deep into the material, and causes small vibrations in the electrons. The vibrations are then passed on from atom to atom, each vibrating at the same frequency as the incoming light source. This makes the light inside the material look bent. Example: a straw in a glass of water.

### Light and matter

The human eye and brain translate light into colour. Light receptors within the eye transmit messages to the brain, producing the familiar sensation of colour. The retina is covered by millions of light-sensitive cells, some shaped like rods and some like cones, and it’s these receptors that process the light and then send this information to the visual cortex. Rods are mostly concentrated around the edge of the retina and transmit mostly black and white information. Cones transmit the higher levels of light intensity that create the sensation of color and visual sharpness. These cells, working in combination with connecting nerve cells, give the brain enough information to interpret and name colours.

Think of atoms like bricks in a wall (chemical compound). Imagine throwing a ball into the wall. If the wall is smooth or has sharp corners, the ball may jump back in different directions. However, if the wall is filled with holes, the ball may go through the wall or get stuck in one of the tricky corners, respectively. Same with every surface when light hits it. The surface may reflect the light back it can absorb light or just let it pass through (transparent things).

This analogy is far from perfect though because light isn’t like a ball. For instance, the light we get to see, called visible light, is only a fraction of the full range of frequencies. A molecule might absorb photons from anywhere across the whole electromagnetic spectrum, from radio waves to X-rays, but it will be colourful only if there is a difference in how strongly it absorbs one visible wavelength over another. As it turns out, this is quite uncommon since most molecules absorb light above the visible spectrum, in the ultraviolet range. So, because electrons in most molecules are bound very tightly, most compounds are white!

Chemical formula or the organic dye indigo. Image: ABC.net.au

Some substances have electrons in the right range of binding strength which makes them suitable to use as dyes. One of the first natural dyes is indigo, commonly used to colour jeans. It derives its colour from a set of three double-bonds at its centre (O=C, C=C, C=O). The problem with indigo and other organic dyes is that it fades away in time because it absorbs energy, instead of reflecting it. In time, bonds break as a result of the damage. Inorganic dyes like pure iron oxide or rust (ochre), however, are lightfast and can last for thousands of years. This is why cave paintings are still visible today!

Lycopene is a bright red carotenoid pigment, a phytochemical found not only in tomatoes but also other red fruits.
Lycopene absorbs most of the visible light spectrum and reflects mainly red back to the viewer, thus a ripe tomato appears red. Image: Colour Therapy Healing

As a conclusion, things do not have color by themselves — only when light (energy) hits them, we can see colors. This is precisely why your surroundings appear greyish or downright black when you’re in the dark. Also, remember our eyes can only see a limited range of colours. But dogs, cats, mice, rats and rabbits have very poor colour vision. In fact, they see mostly greys and some blues and yellows, while bees and butterflies can see colors that we can’t see. Their range of color vision extends into the ultraviolet, and in fact, they couldn’t have survived otherwise. Evolution led bees to adapt ultraviolet vision because flowers leave scatter ultraviolet patterns, allowing the insects to easily identify targets and pollinate. But while humans can’t see colours beyond our visible spectrum, the machines we build can. This is what spectrometers are for.

## How the Brain Perceives Colors?

Color vision is the ability to distinguish different wavelengths of electromagnetic radiation. Color vision relies on a brain perception mechanism that treats light with different wavelengths as different visual stimuli (e.g., colors). Usual color insensitive photoreceptors (the rods in human eyes) only react to the presence or absence of light and do not distinguish between specific wavelengths.

We can argue that colors are not real—they are “synthesized” by our brain to distinguish light with different wavelengths. While rods give us the ability to detect the presence and intensity of light (and thus allow our brain to construct the picture of the world around us), specific detection of different wavelengths through independent channels gives our view of the world additional high resolution. For instance, red and green colors look like near identical shades of grey in black and white photos.

An animal with black and white vision alone won’t be able to make a distinction between, let’s say, a green and red apple, and won’t know which one tastes better before trying them both based on color. Evolutionary biologists believe that human ancestors developed color vision to facilitate the identification of ripe fruits, which would obviously provide an advantage in the competitive natural world.

Why certain wavelengths are paired with certain colors remains a mystery. Technically, color is an illusion created by our brain. Therefore, it is not clear if other animals see colors the same way we see them. It is likely that, due to shared evolutionary history, other vertebrates see the world colored similarly to how we see it. But color vision is quite common across the vast animal kingdom: insects, arachnids, and cephalopods are able to distinguish colors.

What kind of colors do these animals see?

Human color vision relies on three photoreceptors that detect primary colors—red, green, and blue. However, some people lack red photoreceptors (they are “bichromates”) or have an additional photoreceptor that detects somewhere between red and green colors (“tetrachromates”). Obviously, having only 3 photoreceptors doesn’t limit our ability to distinguish other colors.

Each photoreceptor can absorb a rather broad range of wavelengths of light. To distinguish a specific color, the brain compares and quantitatively analyses the data from all three photoreceptors. And our brain does this remarkably successfully—some research indicates that we can distinguish colors that correspond to wavelength differences of just 1 nanometer.

This scheme works in largely the same way in most higher vertebrate animals that have color vision. Although the ability to distinguish between specific shades varies significantly between the species, with humans having one of the best color distinguishing abilities.

However, invertebrates that have developed color vision (and vision in general) completely independently from us demonstrate remarkably different approaches to color detection and processing. These animals can have a exceptionally large number of color receptors. The mantis shrimp, for instance, has 12 different types of photoreceptors. The common bluebottle butterfly has even more—15 receptors.

Does it mean that these animals can see additional colors unimaginable to us? Perhaps yes. Some of their photoreceptors operate in a rather narrow region of light spectrum. For instance, they can have 4-5 photoreceptors sensitive in the green region of the visual spectrum. This means that for these animals the different shades of green may appear as different as blue and red colors appear to our eyes! Again, the evolutionary advantages of such adaptations are obvious for an animal living among the trees and grasses where most objects, as we see them, are colored in various shades of green.

Researchers tried to test if a more complicated set of visual receptors provide any advantages for animals when it comes to the distinguishing between main colors. The findings show that this is not necessarily the case, at least not for the mantis shrimp. Despite the impressive array of receptors detecting light in a much broader part of the electromagnetic spectrum compared to humans, the shrimp’s ability to distinguish between colors that great in comparison to us. However, they determine the colors fast. This is probably more important for practical purposes, as mantis shrimps are predators. A large number of photoreceptors allows for their quick activation at specific wavelengths of light and thus communicate directly to the brain what specific wavelength was detected. In comparison, humans have to assess and quantify the signals from all three photoreceptors to decide on a specific color. This requires more time and energy.

Apart from employing a different number of photoreceptors to sense light of specific wavelengths, some animals can detect light that we humans are completely unable to see. For example, many birds and insects can see in the UV part of the spectrum. Bumblebees, for instance, have three photoreceptors absorbing in the UV, blue, and green regions of the spectrum. This makes them trichromates, like humans, but with the spectral sensitivity shifted to the blue end of the spectrum. The ability to detect UV light explains why some flowers have patterns visible only in this part of the spectrum. These patterns attract pollinating insects, which have an ability to see in this spectral region.

A number of animals can detect infrared light (the long wavelength radiation) emitted by heated objects and bodies. This ability significantly facilitates hunting for snakes that are usually looking for small warm-blooded prey. Seeing them through IR detecting receptors is, thus, a great tool for slow-moving reptiles. The photoreceptors sensitive to IR radiation in snakes are located not in their eye but in “pit organs” located between the eyes and nostrils. The result is still the same: snakes can color objects according to their surface temperature.

As this brief article shows, we humans can see and analyze only a small portion of the visual information available to other creatures. Next time you see a humble fly, think about how different it perceives the same things you are both looking at!

Skorupski P, Chittka L (2010) Photoreceptor Spectral Sensitivity in the Bumblebee, Bombus impatiens (Hymenoptera: Apidae). PLoS ONE 5(8): e12049. doi: 10.1371/journal.pone.0012049

Thoen HH, How MJ, Chiou TH, Marshall J. (2014) A different form of color vision in mantis shrimp. Science 343(6169):411-3. doi: 10.1126/science.1245824

Chen P-J, Awata H, Matsushita A, Yang E-C and Arikawa K (2016) Extreme Spectral Richness in the Eye of the Common Bluebottle Butterfly, Graphium sarpedon. Front. Ecol. Evol. 4:18. doi: 10.3389/fevo.2016.00018

Arikawa, K., Iwanaga, T., Wakakuwa, M., & Kinoshita, M. (2017) Unique Temporal Expression of Triplicated Long-Wavelength Opsins in Developing Butterfly Eyes. Frontiers in Neural Circuits, 11, 96. doi: 10.3389/fncir.2017.00096

## Vision and Art: The Biology of Seeing

Unexpectedly, the most fascinating art book I’ve ever read is written by a Harvard Medical School professor of neurophysiology. “This book is about vision—the process of receiving and interpreting light reflected from objects—and what art reveals about how we see.”

The book starts out with an explanation of light and the basic structure of our vision. Cones are used in daylight. Rods are used in dim light. “It is sometimes incorrectly said that rods are for discriminating luminance and cones are for color. The fact is that luminance (or value) and color are not distinguished along the rod/cone dichotomy. The distinction is made by the next cells in the hierarchy, the retinal ganglion cells…. In short, we see color by subtracting the different cone responses, and we see luminance by summing the different cone responses and the rod responses.”

The author labels two categories of brain functions as the What system and the Where system. The What system deals with object recognition, face recognition, and color perception. The Where system deals with “motion perception, depth perception, figure/ground segregation, and perceiving positional information.”

The book includes a wild illusion demonstrating the effects of equiluminance. The What system can discern the borders between the concentric circles because they are different colors. However, because the circles are the same luminance (or value) the Where system does not. “You should notice a streaming effect in the colored circles. The streaming moves perpendicularly to the high-contrast lines, which induce it.”

Claude Monet also used low luminance contrast to create an illusion of motion. The artist used equiluminant colors in Poppy Field to make the flowers “seem to flow and sway in a breeze.” In The Railway Bridge, equiluminant colors give the river a sense of illusory motion. In Impression: Sunrise, the sun is equiluminant with the sky. Although these effects are much more subtle than the concentric circles, the author modified Sunrise with a brighter, presumably more realistic, sun to compare the effect. “It paradoxically seems less vibrant.”

Monet also experimented with the low luminance contrast using little or no color contrast in Vétheuil in the Fog, which has the opposite effect. “Something defined by very-low-contrast contours is seen by the Where system, but not the What system and may seem to have depth and spatial organization but no clear shape or identity.”

“Painters who use watercolors or pastels… often exploit the low resolution of our color system by applying their color in a looser or blurrier way than the higher-contrast outlines of the objects the color seems to conform to the outlines, even if it actually does not… We think the visual system defines the borders of objects using a high-resolution form system, and then it uses a lower-resolution color system to assign color to the object… Thus, color spreads to fill areas defined by the form system.” This is similar to the way a JPEG file efficiently stores shape and color information, as compared to a resource-consuming bitmap file.

The author briefly mentions Fauvism. “Luminance contrast, not color, is necessary for depth perception. A corollary of this is that you can use any hue you want, as long as you have the appropriate luminance contrast, and still portray three-dimensional shape from shading. This is particularly apparent in the work of the Fauves.”

What color would you expect when you mix yellow and blue—white, green, or gray? The answer depends on whether you are talking about additive, subtractive, or optical mixing.

• Additive – “When you combine red and cyan light you get hueless white. Blue and yellow light also mix to make white, as do any pair of colors of light in which the red-green and blue-yellow opponent activities are balanced. In other words, we see hue only if at least one of the color-opponent channels gives an unbalanced signal.”
• Subtractive – “When blue and yellow pigments are mixed you see only green light that is reflected by both the yellow and blue pigments… With pigments you are combining what absorbs, or subtracts, light.”
• Optical Mixing – “You may be surprised to read that if you mix yellow and blue paint on a palette you get a different color than if you paint tiny dots of yellow and blue, and then stand far enough away that they merge… I walked about 25 feet from the board and saw that the two patches both looked gray, not green. Then I used a toothpick to smear and mix the dots of one of the patches, which resulted in that patch becoming quite green… ‘Optical mixing’ means that adjacent colors blend as if light of the two colors were combined (additive color mixing) rather than like two pigments mixed on a palette (subtractive color mixing)… The Post-Impressionists did indeed achieve optical mixing of colors… The inks in magazine printing blend both additively and subtractively (that is, many of the colored dots are isolated… and therefore combine additively, but others are printed on top of each other… so they blend subtractively.”

“People often contend that black and white are not colors. But, subsequent to the photoreceptor stage, luminance is one of the three axes in color space… you cannot define every color without employing luminance. For example, the difference between brown and yellow or between maroon and pink is solely a difference in luminance, i.e. the position along the black-white axis. So black and white are indeed colors they just don’t have any hue.”

“If you want to see what an artist saw while painting a picture, you should view the painting under the same light he worked in. It works the other way, too: If a painter knows a work will be displayed in bright daylight, he would do well to create it is bright daylight, or he may be surprised by how it looks when displayed a painting hung in a dim corridor may evidence surprisingly bright blues, compared to its appearance in daylight.”

Does Mona Lisa change her expression? “Mona Lisa’s mouth—[when] seen by your peripheral, low-resolution, vision—appears more cheerful than when you look directly at it, when it is seen by your fine-detail fovea… Most of us are not aware of how we move our eyes around or that our peripheral vision can see some things better than our central vision… Facial expressions may be more apparent in the coarse image components than in the finer ones even in real life, because they depend on deep facial muscles, and changes in underlying muscle activity can be effectively blurred by subcutaneous fat. Therefore it may be that our ability to correctly interpret facial expressions in general is better in our peripheral vision than in the center of gaze.”

“Since our eyes view the world from slightly different positions, the images on the two retinas differ slightly. Stereopsis is the ability of the visual system to interpret the disparity between the two images as depth… People whose eyes are misaligned cannot see stereoscopic depth.” The author suggests that this may be an advantage for an artist. “If your visual system is poor at extracting depth, maybe you see the world as flatter than I do, and perhaps you have less trouble ‘flattening’ it onto a piece of paper… Gustav Klimt himself probably was stereoblind. His photograph shows that he was severely cross-eyed.” Studying self-portraits of Rembrandt, she believes he was also stereoblind. “We conclude that poor depth perception is not a detriment to making art and may even by an asset. But you don’t need to poke an eye out in order to be a good artist, since you can get exactly the same effect by closing one eye, which is a common trick taught in art schools to ‘flatten the scene.’”

Some of the many other questions answered in this book include:

• Why does an oil film on water make rainbow colors?
• Why are men more likely to be red/green colorblind than women?
• Why do cats’ eyes appear to glow at night?
• Why do film editors cut on motion?
• Why is equiluminant colored text hard to read?
• Why do we need reading glasses as we age?
• Why should you avoid eye contact with monkeys?

Additional topics include colored shadows, countershading, facial recognition, illusory depth, illusory motion, illusory borders, how color television works, dyslexia, and some bizarre visual disabilities which result from damage to specific parts of the brain.

## Primary Colors Are Red, Yellow and Blue, Right? Well, Not Exactly

Go ahead and ask Google — the knower of all things — to name the primary colors. You'll get a straightforward answer that likely aligns with everything you learned as an elementary school coloring book expert. The primary colors are red, yellow and blue.

But as with most seemingly simple concepts, the answer is actually a whole lot more complex. And while Google isn't exactly lying to you, it doesn't exactly tell the whole story, either.

### What Are Primary Colors?

Here's the deal about primary colors: The players depend on the game. In other words, if you're talking about painting, then yes: Red, yellow and blue are your primary colors. If you're talking about physics and light, though, your primary colors are red, green and blue.

So, what gives? The reason for the confusing contradiction is that there are two different color theories — for "material colors" like the ones used by painters and for colored light. These two theories are known as additive and subtractive color systems.

Stephen Westland, Professor of Colour Science at the University of Leeds in England breaks things down into simple terms (before getting into the confusing complexities), in an email. "We see because light enters our eyes," he says. "Light enters our eyes in two ways: (1) directly from a light source and (2) reflected from an object. This leads to two types of colour mixing, additive and subtractive." [We have retained the British spelling of the word "colour" here.]

"Both systems are accomplishing one task," says Mark Fairchild, professor and director of the Program of Color Science/Munsell Color Science Laboratory at Rochester Institute of Technology in New York. "That is to modulate the responses of the three types of cone photoreceptors in our eyes. Those are roughly sensitive to red, green and blue light. The additive primaries do this very directly by controlling the amounts of red, green and blue light that we see and therefore almost directly map to the visual responses. The subtractive primaries also modulate red, green and blue light, but a little less directly."

Let's get into those distinctions — but fair warning: everything you know about primary colors is about to change before your eyes.

### Additive Color Mixing

Let's talk about the additive system first. When he was 23 years old, Isaac Newton made a revolutionary discovery: By using prisms and mirrors, he could combine the red, green and blue (RGB) regions of a reflected rainbow to create white light. Newton deemed those three colors the "primary" colors since they were the basic ingredients needed to create clear, white light.

"Additive colors are those which make more light when they are mixed together," says Richard Raiselis, Associate Professor of Art at Boston University School of Visual Arts. "A simple way to think about additive light is to imagine three flashlights projecting individual circles of light onto a wall. The shared intersection of two flashlight circles is brighter than either of the circles, and the third flashlight circle intersection will be brighter still. With each mix, we add lightness, therefore we call this kind of mixture additive light." If you imagine each flashlight is fitted with a transparent color filter — one red, one green and one blue — Raiselis says that's the key to understanding additive color mixing.

"When the blue flashlight circle intersects the green one, there is a lighter blue-green shape," he says. "It's cyan. The red and blue mix is lighter too, a beautiful magenta. And the red and green also make a lighter color — and a surprise to nearly everyone who sees it – yellow! So red, green and blue are additive primaries because they can make all other colors, even yellow. When mixed together, red, green and blue lights make white light. Your computer screen and TV work this way. And if you've been onstage, you might have looked up behind the curtain to see the red, green and blue lights that serve as theatre's additive primary colors."

"In simple terms, additive color mixing is where we have a device such as a TV or a smartphone screen that emits light," Westland says. "In most devices, three different colors of light (primaries) are emitted and as they are used they are added together." But the range — or gamut — of colors that can be produced from three additive primaries varies depending upon what the primaries are. Most sources will tell you red, green and blue are the additive primaries, as Newton originally proposed, but Westland says it's a lot more complicated than that.

"It is often mistakenly written that RGB are optimal because the visual system has receptors in the eye that respond optimally to red, green and blue light but this is a misconception," he says. "The long-wavelength sensitive cone, for example, has peak sensitivity in the yellow-green part of the spectrum, not the red part."

### Subtractive Color Mixing

Enter subtractive color. "Subtractive colour mixing results when we mix together paints or inks," Westland says. "It relates to all of the colours we see of non-emissive objects, such as textiles, paints, plastics, inks, etc. "These materials are seen because they reflect the incident light that falls upon them. Take a piece of white paper this paper reflects all of the wavelengths in the visible spectrum to a very high degree. Now add a yellow ink on top of the paper. The yellow ink absorbs the blue wavelengths, leaving the others — which are seen as yellow — to be reflected. So rather than being additive, in this case we start with white (all the wavelengths being reflected) and then start to subtract light at certain wavelengths as we add the primaries."

So the distinction in color systems really comes down to the chemical makeup of the objects involved and how they reflect light. Additive theory is based on objects that emit light, while subtractive deals with material objects like books and paintings. "Subtractive colors are those which reflect less light when they are mixed together," says Raiselis. "When artists' paints are mixed together, some light is absorbed, making colors that are darker and duller than the parent colors. Painters' subtractive primary colors are red, yellow and blue. These three hues are called primary because they cannot be made with mixtures of other pigments."

So, Crayola and Google aren't wrong — in the material world, red, blue and yellow are the primary colors that can be combined to create additional colors of the rainbow. But if you're talking about anything tech-related (as most of us are these days), remember that the primary colors for TVs, computer screens, mobile devices and more, all subscribe to Newton's light-emitting system, so their primary colors are red, green and blue. Kind of. Well, not really.

### The Distinction Between Additive and Subtractive . And Why It's Wrong

"It turns out that if we use three primaries, the best ones to use are cyan, magenta and yellow," Westland says. "Note that these are the primaries that have been identified by the large printing companies who will use CMY (and often black as well) in their commercial devices to make a large range of colors. The idea that the subtractive primaries are red, yellow and blue (RYB) is confusing and should not be taught. It would be wrong to think that cyan and magenta are just fancy names for blue and red."

It's shocking, but true: The names we've been using for our primary colors when it comes to coloring books and paint chips? Totally wrong. "The subtractive primaries are really cyan, magenta, and yellow," Fairchild says. "The names 'blue' for the 'cyan' and 'red' for the 'magenta' are typically misnomers. Other colors can be used as primaries, but they will not produce as wide a range of color mixtures."

The reason behind these inaccurate terms? Light. "The yellow primary controls the amount of blue light reaching our eyes," Fairchild says. "A small amount of yellow primary removes a small amount of blue light from the original white stimulus (e.g. white paper in printing or a white canvas), while a larger amount of yellow removes more blue light. The magenta primary controls the amount of green light and, finally, the cyan primary controls the amount of red light. The subtractive primaries do this by absorbing different amounts of red, green and blue, while the additive primaries simply emit different amounts. It's all about controlling the amounts of red, green and blue light."

Westland offers a scholastic example to illustrate the rampant misconception around primaries. "Imagine you are teaching colour science at school and you explain that the additive primaries are RGB and that the subtractive primaries are RYB," he says. "A particularly bright student asks you: 'why are two of the primaries the same in both systems (R and B) but the G in the additive system is replaced by the Y in the subtractive system?' This is a horrible question because it has no rational answer."

You have to love the candor. The reason for the lack of rationale is that, as we've discussed, red, yellow and blue aren't the real subtractive primaries at all — magenta, yellow, and cyan are. "It turns out that RYB is in fact a particularly poor choice of subtractive primaries," Westland says. "Many of the mixtures that are produced are dull and desaturated and consequently, the gamut of colours you can produce will be small. What you should teach is that there is a clear relationship between the additive and subtractive colour primaries. The optimal additive primaries are RGB. The optimal subtractive primaries are cyan (which is red absorbing), magenta (which is green absorbing), and yellow (which is blue absorbing). Now, there is no conflict between the two systems and, in fact, it can be seen that additive and subtractive primaries are almost mirror images of each other. The best subtractive primaries are CMY because the best additive primaries are RGB."

So, if cyan, magenta and yellow are the real deal primaries when it comes to tactile objects, why does just about everyone on the planet still think the honor belongs to red, blue and yellow? "Well, partly because they are incorrectly taught this from their first days at school," Westland says. "But also because it seems intuitive. It seems intuitive because people believe the following: 1) That it is possible to make all colours by mixing together three primaries, and 2) That the primaries are pure colours that cannot be made by mixing other colours."

So . those beliefs are wrong?

### The Truth About Red and Blue

Well, yes, according to Westland, the idea that three pure primaries can create al the colors in the world is totally false. "We cannot make all colours from three primaries no matter how carefully we choose the primaries," he says. "We cannot do it with additive colour mixing and we cannot do it with subtractive colour mixing. If we use three primaries, we can make all the hues, but we cannot make all the colours we will always struggle to make really saturated (vivid) colours."

Here's the thing: even though we're taught to think of red and blue as "pure" colors, they're simply not. Here's how to prove that: open an art program on your computer and create a red patch on the screen. Then print the patch using a CMYK printer. "The printer will produce red by mixing the magenta and yellow inks that it has," Westland says. "Red can be made by mixing together magenta and yellow. If we use RYB or CMY — or, indeed, almost any other sensible set of three primaries, obviously not three reds! — then we can make all hues however, we cannot make all the colors. But we will get the biggest gamut of colours using CMY and that is why we can say that CMY are the optimal subtractive primaries just as RGB are the optimal additive primaries."

And as far as blue goes, it's not as pure as you think either. "It looks pure because it absorbs strongly in two thirds of the spectrum," Westland says. "It absorbs in the green and red parts. Red absorbs in the blue and green parts. If we mix them together, between them they are absorbing everywhere! The resultant mixture, although it may be a purple colour, will be dull and dark. The absorption spectra of these colours are too broad. It is better to use cyan than blue because cyan absorbs mainly in the red part of the spectrum and magenta absorbs mainly in the green part of the spectrum. If we add magenta and cyan together we get absorbing in the red and green parts of the spectrum but we allow the blue light to be reflected."

To break it down, Westland offers this handy dandy guide:

If this in-depth explanation busted every color myth that's been ingrained in your brain since childhood and you're feeling a bit panicked, take heart: coloring books are reportedly great stress busters. And if you're desperate to learn more, check out Westland's two-minute video series on the subject and his blog. Fairchild also created a great resource that he says is for kids, but honestly — every adult should be required to study it.

If you feel like every person you've dated has cited blue as their favorite color, you're probably not mistaken – apparently, 40 percent of the worldwide population says it's their fave (purple is a close second at 14 percent).

## The Lion King

Researchers in Rochester, New York have found that feeling sad can impact on your ability to identify colours. Participants were shown swatches which had most, but not all, of the colour removed from them and were then asked to identify what colour they were looking at it.

A group who had watched the death of Mufasa in The Lion King found it harder to pick out blue and yellow than others who had not seen the film. Psychologists believe that dopamine – which controls our brain’s reward and pleasure centres – has an impact on how we distinguish these colours.

So while colour might seem to be one of the most straightforward things in our world, it is actually a mystery scientists are only just beginning to unravel.

## Chromatography: Be a Color Detective

Introduction
Do you love to use bright and vibrant colored art supplies such as markers or paints? Do you ever wonder how these colors are made?

The variety of colors comes from colored molecules. These are mixed into the material&mdashwhether ink or paint&mdashto make the product. Some colored molecules are synthetic (or man-made), such as "Yellow No. 5" found in some food dyes. Others are extracted from natural sources, such as carotenoid (pronounced kuh-RAH-tuh-noid) molecules. These are molecules that make your carrot orange. They can be extracted from concentrated natural products, such as saffron.

But there is more to making a color look the way it does in your homemade artwork. You might have learned that many colors, such as orange and green, are made by blending other, "primary" colors. So even though our eyes see a single color, the color of a marker, for instance, might be the result of one type of color molecule or it might be a mix of color molecules responsible. This science activity will help you discover the hidden colors in water-soluble markers.

Background
We see objects because they reflect light into our eyes. Some molecules only reflect specific colors it is this reflected, colored light that reaches our eyes and tells our brains that we are seeing a certain color.

Often the colors that we see are a combination of the light reflected by a mixture of different-color molecules. Even though our brains perceive the result as one color, each of the separate types of color molecules stays true to its own color in the mixture. One way to see this is to find a way to separate out the individual types of color molecules from the mixture&mdashto reveal their unique colors.

Paper chromatography is a method used by chemists to separate the constituents (or parts) of a solution. The components of the solution start out in one place on a strip of special paper. A solvent (such as water, oil or isopropyl alcohol) is allowed to absorb up the paper strip. As it does so, it takes part of the mixture with it. Different molecules run up the paper at different rates. As a result, components of the solution separate and, in this case, become visible as strips of color on the chromatography paper. Will your marker ink show different colors as you put it to the test?

• Two white coffee filters
• Scissors
• Ruler
• Drawing markers (not permanent): brown, yellow and any other colors you would like to test
• At least two pencils (one for each color you will be testing)
• At least two tall water glasses (one for each color you will be testing), four inches or taller
• Water
• Two binder clips or clothespins
• Drying rack or at least two additional tall water glasses (one for each color you will be testing)
• Pencil or pen and paper for taking notes

Preparation

• Carefully cut the coffee filters into strips that are each about one inch wide and at least four inches long. Cut at least two strips, one to test brown and one to test yellow. Cut an extra strip for each additional color you would like to test. How do you expect each of the different colors to behave when you test it with the paper strip?
• Draw a pencil line across the width of each paper strip, about one centimeter from the bottom end.
• Take the brown marker and a paper strip and draw a short line (about one centimeter) on the middle section of the pencil line. Your marker line should not touch the sides of your strip.
• Use a pencil to write the color of the marker you just used on the top end of the strip. Note: Do not use the colored marker or pen to write on the strips, as the color or ink will run during the test.
• Repeat the previous three steps with a yellow marker and then all the additional colors you would like to test.
• Hold a paper strip next to one of the tall glasses (on the outside of it), aligning the top of the strip with the rim of the glass, then slowly add water to the glass until the level just reaches the bottom end of the paper strip. Repeat with the other glass(es), keeping the strips still on the outside and away from the water. What role do you think the water will play?
• Fasten the top of a strip (the side farthest from the marker line) to the pencil with a binder clip or clothespin. Pause for a moment. Do you expect this color to be the result of a mixture of colors or the result of one color molecule? If you like, you can make a note of your prediction now.
• Hang the strip in one of the glasses that is partially filled with water by letting the pencil rest on the glass rim. The bottom end of the strip should just touch the water level. If needed, add water to the glass until it is just touching the paper. Note: It is important that the water level stays below the marker line on the strip.
• Leave the first strip in its glass as you repeat the previous two steps with the second strip and the second glass. Repeat with any additional colors you are testing.
• Watch as the water rises up the strips. What happens to the colored lines on the strips? Does the color run up as well? Do you see any color separation?
• When the water level reaches about one centimeter from the top (this may take up to 10 minutes), remove the pencils with the strips attached from the glasses. If you let the strips run too long, the water can reach the top of the strips and distort your results.
• Write down your observations. Did the colors run? Did they separate in different colors? Which colors can you detect? Which colors are on the top (meaning they ran quickly) and which are on the bottom (meaning they ran more slowly)?
• Hang your strips to dry in the empty glasses or on a drying rack. Note that some colors might keep running after you remove the strips from the water. You might need longer strips to see the full spectrum of these colors. The notes you took in the previous step will help you remember what you could see in case the colors run off the paper strip. Look at your strips. How many color components does each marker color have? Can you identify which colors are the result of a mixture of color components and which ones are the result of one hue of color molecule? Are individual color components brightly colored or dull in color? How many different colors can you detect in total?
• Extra: Most watercolor marker inks are colored with synthetic color molecules. Artists often like to work with natural dyes. It is fairly easy to make your own dye from colorful plants such as blueberries, red beets or turmeric. To make your own dye, have an adult help you finely chop the plant material and place it in a saucepan. And add just enough water to cover the plant material. Let the mixture simmer covered on the stove for approximately 10 to 15 minutes. If, at this point, the color of your liquid is too faint, you might want to remove the lid of the saucepan and continue boiling until some liquid has evaporated and a more concentrated color is obtained. Let it cool and strain when needed. Now you have natural dye. (Handle with caution, as it can stain surfaces and materials.) To investigate the color components of this dye, repeat the previous procedure but replace the marker line with a drop of natural dye. A dropper will help create a nice drop. Let the drop of dye dry before running the paper strip. Would the color of your natural dye be the result of a mixture of color molecules or one specific color molecule? Does the marker of the same color as your natural dye run in a similar way as your natural dye does?
• Extra: In this activity you used water-soluble markers in combination with water as a solvent. You can test permanent markers using isopropyl rubbing alcohol as a solvent. Do you think similar combinations of color molecules are used to color similar-colored permanent markers?
• Extra: You can investigate other art supplies, including paints, pastels or inks in a similar way. Be sure to always choose a solvent that dissolves the material that is being tested to run the chromatography test. Isopropyl rubbing alcohol, vegetable oil and salt water are some examples of solvents used to perform paper chromatography tests for different substances.

Observations and results
Did you find that brown is made up of several types of color molecules, whereas yellow only showed a single yellow color band?

Marker companies combine a small subset of color molecules to make a wide range of colors, much like you can mix paints to make different colors. But nature provides an even wider range of color molecules and also mixes them in interesting ways. As an example, natural yellow color in turmeric is the result of several curcuminoid molecules. The brown pigment umber (obtained from a dark brown clay) is caused by the combination of two color molecules: iron oxides (which have a rusty red-brown color) and manganese oxides (which add a darker black-brown color).

In this activity you investigated the color components using coffee filters as chromatography paper. Your color bands might be quite wide and artistic, whereas scientific chromatography paper would yield narrow bands and more-exact results.

Cleanup
Throw away the paper strips and wash the glasses.

This activity brought to you in partnership with Science Buddies

## Why do we see a different color when we mix two colors? - Biology

Imagine the two of us, arm in arm, looking at a sunset, where the horizon is fretted with golden fire and the deep blue night encroaches from the opposite side of the sky. "What beautiful colours", I say, and you agree.

And then, in the space of the following silence, I am struck by a worry. I can point at the sky and say it is blue, and you will concur. But are you really seeing that blue the way I am seeing it? Perhaps you have just learnt to call what you see "blue", but in actual experience you are seeing nothing like the vivid, rich, blue I see. You are an imposter, calling my blue by the same name as yours, but not really seeing it the way I do. Or, even worse, perhaps I am the one seeing a pale imitation blue, while you see a blue that is infinitely richer and more splendid than mine.

Now I admit that this worry lies in the realm of philosophy, not neuroscience. You might even ask me why I am worrying about this when we could be enjoying the glorious sunset. But when you think about it, it is not clear that I could ever have direct access to what it is like to be you, and you could never have direct access to what it is like to be me, or someone else, or something else, such as a bat. My worry seems more plausible when you consider colour blindness, which affects around 8% of men and half of one percent of women. Many people do not even realise they are colour blind. They live among the colour-seeing, getting by on the fact that there is usually some other difference between things of different colours that they can use to tell them apart, such as differences in shade or texture.

How green is my valley?

Our colour vision starts with the sensors in the back of the eye that turn light information into electrical signals in the brain – neuroscientists call them photoreceptors. We have a number of different kinds of these, and most people have three different photoreceptors for coloured light. These are sensitive to blues, greens and reds respectively, and the information is combined to allow us to perceive the full range of colours. Most colour blind men have a weakness in the photoreceptors for green, so they lose a corresponding sensitivity to the shades of green that this variety helps to distinguish.

At the other end of the scale, some people have a particularly heightenedsensitivity to colour. Scientists call these people tetrachromats, meaning “four colours”, after the four – rather than three – colour photoreceptors they possess. Birds and reptiles are tetrachromatic, and this is what allows them to see into the infrared and ultraviolet spectra. Human tetrachromats cannot see beyond the normal visible light spectrum, but instead have an extra photoreceptor that is most sensitive to colour in the scale between red and green, making them more sensitive to all colours within the normal human range. To these individuals, it is the rest of us who are colour blind, as while most of us would be unable to easily distinguish an exact shade of summer-grass-green from Spanish-lime-green, to a tetrachromat it would seem obvious.

So yes, as we share this sunset, perhaps I am seeing something you cannot see, or you are seeing something I cannot see. If our colour vision is wired differently, the information going in could be more or less the same between us. But as you tell me this, with the sun sinking slowly below the horizon, you can sense that it has not really helped with my true worry. I am worried – and perhaps you are too – that although we both have the same machinery in our eyes and we are both able to see the green of the trees, the red of the sun and the blue of the sky, that when I say "blue", it creates an inner experience that differs from yours when you say "blue".

Behind blue eyes

My worry about your inner perception of the colour blue is a facet of the basic isolation that is part of the human condition. Even if we think we can really know other people, we cannot be certain of that knowledge. Historically, psychologists have adopted a stance called behaviourism, which acts as if questions about inner experience are irrelevant. This approach states that if you call my blue "blue", and you can always tell it from red, and if we both know it is the correct colour for the sky, my eyes and the Smurfs, then who cares what the inner experience is?

There is a lot of mileage in this perspective, but maybe there is also some wisdom in trying to convince ourselves that the difference between our inner experiences is real, and does matter – and, in fact, that some difference is inevitable. We use common words, and use them to refer to shared experiences, but nobody can see the same sunset, merely because perception is a property of the person, not of the sunset. Because there is something that it is like to be you, and your “you-ness” is unique, we are certainly seeing different things when we talk about looking at something blue, if only because the act of seeing incorporates feelings and memories, as well as the raw light information arriving at our eyes.

In any case, the sun has set and we walk away. You might be seeing a richer blue in the sunset than me, but you will not have the same memories of the other sunsets I have seen and the people I have watched them with. We could get our vision tested and find out who was better at perceiving colours, but we would never know what it was like to be the other person seeing a particular colour. As long as we can both say that it is a beautiful sunset, we can agree and be secure in the knowledge that I see my blue, and you see your blue, and although we may not see the exact same thing, we have shared it. And that sharing is itself unique to you and me, because no two other people in the world have the same two minds.

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## Why do we see a different color when we mix two colors? - Biology

If you mix red, green, and blue light, you get white light. Red, green, and blue (RGB) are referred to as the primary colors of light. Mixing the colors generates new colors, as shown on the color wheel or circle on the right. This is additive color. As more colors are added, the result becomes lighter, heading towards white. RGB is used to generate color on a computer screen, a TV, and any colored electronic display device.

When you mix colors using paint, or through the printing process, you are using the subtractive color method. The primary colors of light are red, green, and blue. If you subtract these from white you get cyan, magenta, and yellow. Mixing the colors generates new colors as shown on the color wheel, or the circle on the right. Mixing these three primary colors generates black. As you mix colors, they tend to get darker, ending up as black. The CMYK color system (cyan, magenta, yellow, and black) is the color system used for printing.

Experiment with this RGB color mixer to get a feel for the effect of mixing the three different additive primary colors. The test box beside each slider shows the relative proportions of red, blue and green on a scale from 1 to 255. The sliders themselves show the appearance of the individual colors for your selected color. Notice how the resulting color compares with pigment-based mixing  the effects are very different.

Mixing colors of light and mixing colors of paint produce very different results.

## Night Vision And Humans: Why Can't We See Color?

When we are in a fairly dark room, or outside at night away from lights, we can still see, but we can't see the colors of things very well. Why is that?

### Sensing Light

There are two kinds of light-sensitive organs located in the backs of our eyes: rod-shaped and cone-shaped. Both rods and cones are sensitive to light. The difference between them is that the rods allow us to see in very dim light but don't permit detection of color, while the cones let us see color but they don't work in dim light.

When it gets dark the cones lose their ability to respond to light. The rods continue to respond to available light, but since they cannot see color, so to speak, everything appears to be various shades of black and white and gray.

### Dim Light

A curious thing is that in dim light you can see more clearly out of the side of your eye, because the light-sensitive rods are more highly concentrated off to the side in the back of your eye.

So, next time you're out on a clear night, notice how little color you can see, and how you can see objects like dim stars better out of the corner of your eye than from the center.