# Why three primary colors? And why red, yellow, and blue? **URL:** \ **Category:** Factual Questions\ **Created:** [November 3, 2017, 9:03am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583 "2017-11-03T09:03:08Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![PlaceboTarget](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/placebotarget/32/1073_2.png) [@PlaceboTarget](https://boards.straightdope.com/u/PlaceboTarget)\ **Post date:** [November 3, 2017, 9:03am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/1 "2017-11-03T09:03:08Z")
I was wondering if there is a mathematic(-like) reason why there should be 3 primary colors and why these are red, yellow and blue. ---
**Author:** ![AHunter3](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ahunter3/32/368_2.png) [@AHunter3](https://boards.straightdope.com/u/AHunter3)\ **Post date:** [November 3, 2017, 10:09am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/2 "2017-11-03T10:09:01Z")
I think that what there is, instead, is a biological reason. We have cones that are keyed to those colors. In other words, they’re not intrinsically primary, they’re primary _to us_. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 3, 2017, 10:09am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/3 "2017-11-03T10:09:28Z")
> [@PlaceboTarget](#): > > I was wondering if there is a mathematic(-like) reason why there should be 3 primary colors and why these are red, yellow and blue. Discounting the rods, which are of little use in daytime color vision, the human eye has three types of sensor: the α-, β-, and γ-cones. For this reason, human-perceived color is naturally treated as a three-coordinate system. There is flexibility about _which_ three coordinates to use. Red/Green/Blue are the common choice for the additive colors of a computer monitor; Cyan/Magenta/Yellow are the common choice for the “subtractive” colors of printing. Y/I/Q (and related Y/U/V systems) were introduced for compatible color TV broadcasting and, since human brain has high spatial resolution only for the luminance coordinate, is used in systems like Jpeg and Mpeg for better compression. Although the initial responses in human retina are from the three cone types, by the time color information gets to the optic nerve the data has been transformed into three coordinates that can very roughly be described as Yellow (or Red plus Green, or White), Red minus Green, and Blue minus Yellow. Ignoring luminance (brightness), human-perceived color is described [with two dimensions](https://upload.wikimedia.org/wikipedia/commons/3/3b/CIE1931xy_blank.svg). The curved triangle in the image shows the 2-D “gamut” of chromaticities that can be perceived by a typical human. The “gamut” producible by a 3-phosphor monitor will be the triangle formed by the three phosphor colors. Note that substituting Green for the Yellow in OP’s example will enlarge the gamut significantly. ---
**Author:** ![elfkin477](https://avatars.discourse-cdn.com/v4/letter/e/a9a28c/32.png) [@elfkin477](https://boards.straightdope.com/u/elfkin477)\ **Post date:** [November 3, 2017, 10:14am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/4 "2017-11-03T10:14:37Z")
Red, yellow, and blue can be combined in various ways to create all the other colors. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 3, 2017, 10:27am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/5 "2017-11-03T10:27:15Z")
More of a biological reason: humans normally have trichromatic color vision enabled by three types of cone cells, each kind sensitive to a different range of wavelengths. Therefore there is a three-dimensional space of different visible colors. Now suppose you wish to reproduce colors by mixing pigments. You need at least three to cover a good portion of a three-dimensional space, but red, yellow and blue are not your only choices (indeed, which red, which yellow, and which blue?) The idea is that each paint absorbs a different spectrum of wavelengths of light, so different mixtures result in different colors. A popular color printing process called [CMYK](https://en.wikipedia.org/wiki/CMYK_color_model) actually uses 4 colors: cyan, magenta, yellow, and black, and produces a decent range of colors. The first three are roughly blue, red, and yellow. ETA ninjaed… ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 3, 2017, 10:30am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/6 "2017-11-03T10:30:58Z")
> [@elfkin477](#): > > Red, yellow, and blue can be combined in various ways to create all the other colors. As I just tried to explain in the discussion of the “gamuts” of monitors, a Green with much saturation can NOT be produced by any combination of red, yellow and blue light added together … unless you add a NEGATIVE amount of red light to Yellow (or Blue plus Yellow)! ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 3, 2017, 10:37am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/7 "2017-11-03T10:37:54Z")
I believe the red, yellow, and blue refer to mixing paints (subtractive color), as I described in my post. I don’t think anyone would try to reproduce green light by combining red and yellow. Indeed, devices like LED screens typically have RGB pixels. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [November 3, 2017, 10:55am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/8 "2017-11-03T10:55:26Z")
> [@DPRK](#): > > I believe the red, yellow, and blue refer to mixing paints (subtractive color), as I described in my post. I don’t think anyone would try to reproduce green light by combining red and yellow. Indeed, devices like LED screens typically have RGB pixels. As a rule, you can create any color in that diagram **septimus** linked to that lies within the boundaries of the polygon created by your source colors. If you only have two source colors, those are represented by two points on the Color Diagram, and you can create any color on that line. A line connecting a red point and a yellow point doesn’t extend anywhere into the green, so you can’t make green. Color monitors have three different colors because you can span most of the color diagram with a big triangle, but you have to have one of the points in the greenish area, or else you’ll miss a lot of the chart. (One problem of the chart is that it doesn’t represent colors “equally”. The regions of colors which aren’t distinguishable form really big ellipses (Macadam ellipses) up at the top, and much smaller elipses below. There have been various schemes to re-organize the chart into regions of equal uncertainty. I’ve also seen attempts to create viewers that give a more faithful representation of color. One Japanese patent used something like 18 different LEDs to more faithfully span the color space. You could make colors with it that you can’t on a standard 3-color monitor. ---
**Author:** ![Isamu](https://avatars.discourse-cdn.com/v4/letter/i/7c8e57/32.png) [@Isamu](https://boards.straightdope.com/u/Isamu)\ **Post date:** [November 3, 2017, 11:21am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/9 "2017-11-03T11:21:41Z")
> [@septimus](#): > > Ignoring luminance (brightness), human-perceived color is described [with two dimensions](https://upload.wikimedia.org/wikipedia/commons/3/3b/CIE1931xy_blank.svg). The curved triangle in the image shows the 2-D “gamut” of chromaticities that can be perceived by a typical human. The “gamut” producible by a 3-phosphor monitor will be the triangle formed by the three phosphor colors. Note that substituting Green for the Yellow in OP’s example will enlarge the gamut significantly. Thanks for that. The x and y axes in that image are numbered, what are they measuring? ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 3, 2017, 12:09pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/10 "2017-11-03T12:09:12Z")
Just as 2-D space leads to four cardinal directions: {North, South, East, West} or {Left, Right, Up, Down} so the two dimensions of color hue lead to the four Primary colors {Red, Green, Blue, Yellow}. This thread’s emphasis on color Threeness ignores that the Four primary colors of nursery school are, for many purposes, the best set to think about! ~ ~ ~ ~ ~ ~ ~ [from the computer folklore desk] This discussion of monitor gamuts triggered my memory. Sometime in the mid 1980’s — though I can’t put a face or company name to it — I became aware of a proposal to build an LED-based display using red, green, and yellow LED’s! (Blue LEDs didn’t exist then, at least commercially.) I don’t think the project ever got off the ground. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 3, 2017, 12:12pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/11 "2017-11-03T12:12:51Z")
> [@Isamu](#): > > Thanks for that. The x and y axes in that image are numbered, what are they measuring? Short answer: [CIE 1931](https://en.wikipedia.org/wiki/CIE1931). From the physical definition, as well as by inspecting the chart, one can see the effect of the parameters x and y, but it would be a misleading oversimplification to try to make a statement along the lines “x is red; y is green.” ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 3, 2017, 12:23pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/12 "2017-11-03T12:23:45Z")
> [@septimus](#): > > [from the computer folklore desk] This discussion of monitor gamuts triggered my memory. Sometime in the mid 1980’s — though I can’t put a face or company name to it — I became aware of a proposal to build an LED-based display using red, green, and yellow LED’s! (Blue LEDs didn’t exist then, at least commercially.) I don’t think the project ever got off the ground. Searching Wikipedia leads to a claim that [Sharp Aquos](https://en.wikipedia.org/wiki/Sharp_Aquos) has this [ETA has RGBY, at least some models], though that model would not have been around in 1980. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 3, 2017, 12:25pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/13 "2017-11-03T12:25:24Z")
> [@Isamu](#): > > Thanks for that. The x and y axes in that image are numbered, what are they measuring? The [Wikipedia article](https://en.wikipedia.org/wiki/CIE_1931_color_space) gives some math details but I think it’s simplest just to treat the (x, y, z) and (X, Y, Z) spaces as arbitrary. You can Google to find formulae to convert to (r,g,b) or whatever as the need arises. The numbers around the curved portion of the almost-triangular gamut are the wavelengths of spectral (pure) colors. I’ve read that it was Sir Isaac Newton himself who was the first to think of turning the rainbow sequence into a “closed loop” by adding Purple between Red and Violet, thus forming a _color wheel_ — is this true? ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [November 3, 2017, 1:35pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/14 "2017-11-03T13:35:43Z")
The way I’ve read about the geometrical conceptual phenomenological space of color was as [a spindle](https://books.google.com/books?id=FSLFjw1EcBwC&pg=PA10&lpg=PA10&dq=the+color+spindle&source=bl&ots=jt68TwS7UD&sig=YxIPUui4V9DdAb6wg-EBN4z5Wuk&hl=en&sa=X&ved=0ahUKEwix-LjVuKLXAhUG42MKHbc2DUcQ6AEITTAL#v=onepage&q=the%20color%20spindle&f=false), as in the Swedish Natural Color System (NCS). It may be outdated given the monitor and gamut discussions above. Within that framework though … Hue is by polar coordinates. Chromaticness and brightness are two linear coordinatess. Together they make up a spindle. The section in that chapter also references other geometric models of color with some interesting compare/contrast. Questions. Is hue perception by those with dichromatic vision (“color blindness”) still well described in a polar system? Would perception by creatures with greater than three hue receptors be described better by something other than polar coordinates? Or more broadly put, would the number of hue receptors a viewer possesses impact how to best model the phenomenological space? If so in what ways? The _evolution_ of color vision and consequently the why we’ve developed the perceptions we have are comprehensively discussed [here](http://www.sciencedirect.com/science/article/pii/S0042698908003222) if anyone wants a read. Some sample bits … > [@](#): > > … To investigate the spectral location as well as the number of receptors Vorobyev (1997a) put forward a model that uses accuracy of reconstruction of reflectance spectra from photoreceptor quantum catches as the performance criterion. This predicts that for realistic levels of receptor noise the complement of receptors present in (standard) trichromatic primates is optimal for reconstructing flower and fruit spectra in the human visible range (400–700 nm). Decreasing the number of cone spectral types from three to two significantly decreases the accuracy of reconstruction, while increasing the number of cones to four would not improve accuracy. Similarly, tetrachromatic vision of birds is optimal for reconstructing the reflectance spectra within the bird visible range (300–700 nm): decreasing the number of cones from four to three significantly decreases the accuracy of reconstruction, while increasing the number of cones from four to five has practically no benefit. … > > … the fact that primates have overlapping spectral sensitivities may indicate that their colour vision is evolutionary adapted to tasks different from those of other animals (Vorobyev, 2004). … ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 3, 2017, 2:54pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/15 "2017-11-03T14:54:38Z")
Although it’s a complete hijack to the thread, those who’ve not seen it before may forgive me for [linking to a picture of a hamburger](http://www.t-a-y-l-o-r.com/land/) based on an amazing discovery by Edwin Land. The hamburger almost looks real — it even has **green lettuce!** — but the picture is created with TWO colors, not three. Throughout the image the Green and Blue pixel values are always essentially identical to each other so the image is built strictly from Red and White. The human eye can reconstruct colors that aren’t really there! When Dr. Land demonstrated this effect by projecting red and white light onto a screen, I think the effects were much more impressive than this hamburger picture. ---
**Author:** ![pulykamell](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/pulykamell/32/3166_2.png) [@pulykamell](https://boards.straightdope.com/u/pulykamell)\ **Post date:** [November 3, 2017, 3:06pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/16 "2017-11-03T15:06:59Z")
> [@septimus](#): > > Although it’s a complete hijack to the thread, those who’ve not seen it before may forgive me for [linking to a picture of a hamburger](http://www.t-a-y-l-o-r.com/land/) based on an amazing discovery by Edwin Land. > > The hamburger almost looks real — it even has **green lettuce!** — but the picture is created with TWO colors, not three. Throughout the image the Green and Blue pixel values are always essentially identical to each other so the image is built strictly from Red and White. The human eye can reconstruct colors that aren’t really there! > > When Dr. Land demonstrated this effect by projecting red and white light onto a screen, I think the effects were much more impressive than this hamburger picture. To me, it looks like a bunch of pink/red and gray, but I could see how the lettuce bleeds into green, when contrasted against the red of the tomato. I think it’s somewhat similar to the “what color is the dress” viral post that went around about a year or two ago. The brain is very good at filling in the gaps of color and “color correcting” where applicable. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [November 3, 2017, 3:16pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/17 "2017-11-03T15:16:37Z")
Yeah, the hamburger brown looks brown enough when my eyes adjust for the red-only filter, but the lettuce doesn’t even look like lettuce, much less green. The other colors are shades of red and white and are unsurprising. ---
**Author:** ![DSeid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/dseid/32/20194_2.png) [@DSeid](https://boards.straightdope.com/u/DSeid)\ **Post date:** [November 3, 2017, 3:39pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/18 "2017-11-03T15:39:22Z")
Oh I perceive the “lettuce” as green. How much of that though is due to imposed expectation and how much due to Opponent Process effects? Our minds are primed to see that which is expected to be seen and will perceive it even when it is not really there. Same sort of process that makes proofreading what you have typed so difficult: you expect to see the word you meant to type there and perceive its presence, read the sentence typed as having it, even when it is not there. Some of the effect may be that we expect lettuce to be green. But there is a red green opponent process that may be occurring. It can be experienced in the “green dot illusion” [here](https://www.youtube.com/watch?v=DgaC3c3NBds) and is explained [here](http://the-mouse-trap.com/2006/07/13/the-green-dot-illusion-and-opponent-process-theory/): > [@](#): > > … the moving green dot illusion works because red and green are opponent processes. When pink (which may be conceived as low-saturation red) dots are present in the visual field, then for that portion of visual filed, Red Qualia is exaggerated and Green Qualia inhibited further down the visual pathways. Prolonged presence of Red stimuli ensures that there is no need to keep inhibiting ‘green qualia’ as habituation happens and as the Red signal is strong and continuous one so the need to inhibit Green does not arise. If one refers to the gated dipole opponent process theory of Grossberg, then it is apparent that due to the gating of the dipole, when the RED stimuli disappears from the on-channel then the ‘off channel’ (corresponding to Green qualia) would result in a sudden rebound and thus momentarily the Green qualia would be perceived. Here it is instructive to note that the signal is R minus G i.e Red is the presence of signal and green the absence (or below threshold or negative signal). Thus the green dot illusion would become more stronger if pure red is used and a similar illusion can be produced by moving yellow dot when blue dots are involved. ---
**Author:** ![septimus](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/septimus/32/410_2.png) [@septimus](https://boards.straightdope.com/u/septimus)\ **Post date:** [November 3, 2017, 3:50pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/19 "2017-11-03T15:50:03Z")
Yes, I think you have to see it reflected off a screen from two projectors before the _effect becomes stunning_. (Note also that a background light — e.g. if you neglected to extinguish ceiling lamps when viewing the gif — will spoil the effect. Directly if a colored reflective object is in view, but even pure white will spoil unless your monitor and lightings are all producing the same white as Mean Local Sunlight.) Such demonstrations would have a variety of convincing greens and blues, with human cortex adapting chrominance locally, or as fovea flickers. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 3, 2017, 3:58pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/20 "2017-11-03T15:58:36Z")
IIRC there is some evidence that, under certain circumstances, the human eye can reconstruct colors that aren’t really there— as in truly impossible— the article on [impossible colors](https://en.wikipedia.org/wiki/Impossible_colors) describes some experiments but does not go into technical detail. This seems to fall into the same category of optical illusions mentioned above. [Next page](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583.md?page=2)