# 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:** 15\ **Page:** 2
**Author:** ![filmore](https://avatars.discourse-cdn.com/v4/letter/f/7993a0/32.png) [@filmore](https://boards.straightdope.com/u/filmore)\ **Post date:** [November 3, 2017, 4:01pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/21 "2017-11-03T16:01:16Z")
> [@elfkin477](#): > > Red, yellow, and blue can be combined in various ways to create all the other colors. I think it is a simplification to say that mixing colors can “make” another color. It’s probably more correct to say that mixing two colors fools our brain into thinking it’s another color. The two colors excite the rods in a similar way as the pure color does, so our brain can’t tell the difference. It thinks yellow+blue light hitting our eye is the same pure green. Some people have 4 cones. Are they able to see the separate colors better? That is, is their eye not fooled into seeing green when yellow+blue are together. ---
**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, 5:50pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/22 "2017-11-03T17:50:21Z")
> [@filmore](#): > > I think it is a simplification to say that mixing colors can “make” another color. It’s probably more correct to say that mixing two colors fools our brain into thinking it’s another color. … Well that does end up coming down to the question of what “color” actually is. Is “color” the objective property, the wavelength of the light? Or is it the qualia, the mind’s experience? And while that risks going more into GD than GQ territory the question does highlight that color perception is an active process, from the beginning of the input being a the result of the same inputs differentially triggering different populations of cone receptors, to higher level processing based on context and priming from higher level prototypes downstream. Color is arguably _always_ what the brain thinks is there. ---
**Author:** ![Marvin\_the\_Martian](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/marvin_the_martian/32/2898_2.png) [@Marvin\_the\_Martian](https://boards.straightdope.com/u/Marvin_the_Martian)\ **Post date:** [November 3, 2017, 6:40pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/23 "2017-11-03T18:40:27Z")
> [@DSeid](#): > > Well that does end up coming down to the question of what “color” actually is. > > Is “color” the objective property, the wavelength of the light? Or is it the qualia, the mind’s experience? There is a whole range of colors (those between red and blue on the “color wheel”) which do not correspond to a single wavelength of light (pink is an example - there is no pink in a rainbow). Also, as has been stated in other posts, there are many combinations of wavelengths/intensities that will cause the eye to perceive the same color. Therefore I would argue that color is in the mind of the beholder. ---
**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, 6:57pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/24 "2017-11-03T18:57:16Z")
> [@filmore](#): > > It thinks yellow+blue light hitting our eye is the same pure green. Yellow and blue light actually make white, not green. Yellow and blue pigments make green. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [November 3, 2017, 7:17pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/25 "2017-11-03T19:17:23Z")
> [@AHunter3](#): > > 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_. Well, our cones are responsive to a range of wavelengths, but the wavelengths that they best respond to aren’t R, G, B. The red cone responds best to yellowish green, the green to greenish yellow, and the blue to violet blue. > [@Isamu](#): > > Thanks for that. The x and y axes in that image are numbered, what are they measuring? You may consider them arbitrary. There is also a z axis (which is usually called “Y”, as opposed to the vertical “y” axis for reasons I won’t get into). They are based on a \*\*transformation \*\*from numbers **approximating but not equal to** the cone responses. > [@DPRK](#): > > 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. We’ve had a couple threads about the Aquos. The consensus seems to be that it is marketing BS. ---
**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, 7:52pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/26 "2017-11-03T19:52:40Z")
> [@thelurkinghorror](#): > > Well, our cones are responsive to a range of wavelengths, but the wavelengths that they best respond to aren’t R, G, B. The red cone responds best to yellowish green, the green to greenish yellow, and the blue to violet blue… Probably best not to refer to them as “red”, “green”, or “blue” receptors, and the one that responds strongest to red, the L receptor, peaks not at “yellowish green” but more yellow to maybe yellow orange. The M is pretty solidly peaking at green. What is most notable however is the huge overlap between the wavelengths that those two differentially respond to with the brain interpreting the degree of response in each population together as the precise color that is experienced. The result is pretty good discrimination through the large range that they cover, but less discrimination between wavelengths in the peak range of the S receptor that peaks at about 440. ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [November 3, 2017, 8:24pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/27 "2017-11-03T20:24:07Z")
> [@DSeid](#): > > Probably best not to refer to them as “red”, “green”, or “blue” receptors, and the one that responds strongest to red, the L receptor, peaks not at “yellowish green” but more yellow to maybe yellow orange. The M is pretty solidly peaking at green. What is most notable however is the huge overlap between the wavelengths that those two differentially respond to with the brain interpreting the degree of response in each population together as the precise color that is experienced. The result is pretty good discrimination through the large range that they cover, but less discrimination between wavelengths in the peak range of the S receptor that peaks at about 440. Well not everybody has the same peak wavelength sensitivity, so the actual color that provides the best response ranges, but is still pretty narrow. Also keep in mind that conversion from wavelength to RGB or another system is an inexact process and experiences oversaturation (using [tool here](https://academo.org/demos/wavelength-to-colour-relationship/)): L: 564 to 580; approx yellow-green rgb(207,255, 0) to yellow rgb(255,255, 0)\* M: 534 to 545; approx green rgb(108,255, 0) to green-yellow rgb(146,255, 0) S: 420 to 440; approx violet with a hint of blue rgb(106,0, 255) to blue rgb(0,0, 255) If you prefer, look at the wavelengths along the edge of the [CIE horseshoe](https://upload.wikimedia.org/wikipedia/commons/3/3b/CIE1931xy_blank.svg). \*From there, you might say that 580nm is more orangish yellow. ---
**Author:** ![Alessan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/alessan/32/465_2.png) [@Alessan](https://boards.straightdope.com/u/Alessan)\ **Post date:** [November 3, 2017, 11:10pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/28 "2017-11-03T23:10:51Z")
[Very relevant xkcd.](https://www.xkcd.com/1882/) ---
**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, 11:42pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/29 "2017-11-03T23:42:51Z")
> [@thelurkinghorror](#): > > Well not everybody has the same peak wavelength sensitivity, so the actual color that provides the best response ranges, but is still pretty narrow. Also keep in mind that conversion from wavelength to RGB or another system is an inexact process and experiences oversaturation (using [tool here](https://academo.org/demos/wavelength-to-colour-relationship/)): > > L: 564 to 580; approx yellow-green rgb(207,255, 0) to yellow rgb(255,255, 0)\* > M: 534 to 545; approx green rgb(108,255, 0) to green-yellow rgb(146,255, 0) > S: 420 to 440; approx violet with a hint of blue rgb(106,0, 255) to blue rgb(0,0, 255) > > If you prefer, look at the wavelengths along the edge of the [CIE horseshoe](https://upload.wikimedia.org/wikipedia/commons/3/3b/CIE1931xy_blank.svg). > \*From there, you might say that 580nm is more orangish yellow. Yes, better tools than the linear one I was looking at and using it I’d go with L being centered more as yellow than greenish-yellow _or_ orangish-yellow (although more so on the latter than the former tool) and the M centered on fairly solidly on green on both tools. YMMV. Still the wide overlap of differential response peaking in the greens to yellow-oranges remains the feature that stands out. As the evolution of color perception article points out, primates do particularly well at discriminating between colors common in plants and their fruit. Blues just did not have the same discriminatory salience. The article though includes flowers but to me it is notable that we do not even perceive some of the shorter wavelengths that birds’ and many insects’ visual systems react to and that are often very key in discriminating patterns on flowers of relevance to them (patterns that our visual systems sometimes do not respond to at all and certainly not with any discriminatory capacity). And **Alessan** … we fail. No hyperdimensional four-sided quantum Klein manifold! ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [November 4, 2017, 1:46am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/30 "2017-11-04T01:46:48Z")
> [@DSeid](#): > > Yes, better tools than the linear one I was looking at and using it I’d go with L being centered more as yellow than greenish-yellow _or_ orangish-yellow (although more so on the latter than the former tool) and the M centered on fairly solidly on green on both tools. YMMV. As the difference between L and M is only ~30 nm, it is possible to have both, but inherit 2 that are very similar, and may lead to poor discrimination in some cases. This is the inherited color blindness protanomaly and deuteranomaly. > [@](#): > > Still the wide overlap of differential response peaking in the greens to yellow-oranges remains the feature that stands out. As the evolution of color perception article points out, primates do particularly well at discriminating between colors common in plants and their fruit. Blues just did not have the same discriminatory salience. Which article? That theory is disputed, though still I think the dominant hypothesis. > [@](#): > > The article though includes flowers but to me it is notable that we do not even perceive some of the shorter wavelengths that birds’ and many insects’ visual systems react to and that are often very key in discriminating patterns on flowers of relevance to them (patterns that our visual systems sometimes do not respond to at all and certainly not with any discriminatory capacity). Even if we had the mechanisms to detect these wavelengths, they would never reach our retina. Our lens filters out a bunch of bluish light. People who have cataract surgery get their removed, and they can sometimes see into the near UV range slightly. ---
**Author:** ![DPRK](https://avatars.discourse-cdn.com/v4/letter/d/4491bb/32.png) [@DPRK](https://boards.straightdope.com/u/DPRK)\ **Post date:** [November 4, 2017, 1:48am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/31 "2017-11-04T01:48:57Z")
I recall there was a bit of news about an [artist with trained tetrachromatic vision](http://www.bbc.com/future/story/20140905-the-women-with-super-human-vision). She was understandably demanding and discriminating about mixtures of paint and shades of color. Three is a bare minimum for color reproduction, but any artist has a lot more than that on her palette. (Not that there is anything wrong with grisaille!) ---
**Author:** ![thelurkinghorror](https://avatars.discourse-cdn.com/v4/letter/t/7c8e57/32.png) [@thelurkinghorror](https://boards.straightdope.com/u/thelurkinghorror)\ **Post date:** [November 4, 2017, 2:04am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/32 "2017-11-04T02:04:29Z")
Never mind, I saw what article you were referring to. I somehow skipped over this below post. > [@DSeid](#): > > 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. The most common form people encounter this in is the HSV or HSL systems (only slight differences between the two). Any computer graphics program uses these values, including lowly MSPaint (as Hue, Sat, Lum). The conversion between RGB and HSL is fairly simple. CIE 1931 has been mostly supplanted by CIE 1964 versions, of which there are two competing systems (CIELUV and CIELAB). Those can also be easily converted to cylindrical systems if needed. > [@](#): > > 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? If by polar you mean something like HSL, then no. But when talking about physiological responses, we would use an opponent system which has poles but no cylindrical component.. In the common forms of color blindness where red and green are confused, the opponent channel that measures the difference between the two is not providing responses well, but the one that measures the sum of L+M vs. the S response works fine With 4+ receptors, you would add another dimension, but we find it difficult to think or represent things in 4-dimensional space. ---
**Author:** ![asterion](https://avatars.discourse-cdn.com/v4/letter/a/4da419/32.png) [@asterion](https://boards.straightdope.com/u/asterion)\ **Post date:** [November 4, 2017, 5:16am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/33 "2017-11-04T05:16:02Z")
> [@DPRK](#): > > 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. Sharp was pushing it for a while a couple years ago. Big ad campaign with George Takei. They still might be releasing models, but I never saw anything that made me think that RGBY was much more than a marketing gimmick. ---
**Author:** ![Melbourne](https://avatars.discourse-cdn.com/v4/letter/m/b5e925/32.png) [@Melbourne](https://boards.straightdope.com/u/Melbourne)\ **Post date:** [November 4, 2017, 9:05am UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/34 "2017-11-04T09:05:30Z")
> [@DPRK](#): > > I recall there was a bit of news about an [artist with trained tetrachromatic vision](http://www.bbc.com/future/story/20140905-the-women-with-super-human-vision). She was understandably demanding and discriminating about mixtures of paint and shades of color. Three is a bare minimum for color reproduction, but any artist has a lot more than that on her palette. (Not that there is anything wrong with grisaille!) She’s a “trained artist”, but there is another profession where women with good color perception excel – color matching vehicle paint repair jobs. Good working conditions, no heavy lifting, and on average, women are just better at it than blokes. Biological advantage. ---
**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 4, 2017, 1:49pm UTC](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583/35 "2017-11-04T13:49:37Z")
> [@thelurkinghorror](#): > > … If by polar you mean something like HSL, then no. But when talking about physiological responses, we would use an opponent system which has poles but no cylindrical component… In the common forms of color blindness where red and green are confused, the opponent channel that measures the difference between the two is not providing responses well, but the one that measures the sum of L+M vs. the S response works fine > > With 4+ receptors, you would add another dimension, but we find it difficult to think or represent things in 4-dimensional space. By polar I mean nothing more than polar coordinates, and if by “something like HSL” you mean like hue is represented there, yes. Thinking about it I think more receptors (say tetrachromic like in bird, which both extends the blue into UV and allows for better blue discriminations) would not require changing the description of hue as along polar coordinates, nor introduce higher level dimensionality. Although the concept of describing color physiologically as the activation level of each physiologic channel is very interesting. Before trying to engage on the description of color along physiologic dimensions though, I’d like to explain that my question more comes from the book I had quoted the information on the NCS system from, which was “The Geometry of Thought.” The major thesis of that book was describing concepts in geometric terms and by implication that much thinking about concepts and analogy-making could be thought of as geometric transformations. In the color section part of the discussion was that our descriptions of skin color seems to map as its own color spindle occupying a smaller region of the larger color spindle but with the skin tones of those labeled as “Black, White, Yellow, Red …” (which of course are in fact shades of brown, peachy, etc.) occupying in similar positions on that smaller spindle that they do on the larger one. Ever since reading that years ago I’ve been fascinated by the idea of modeling analogy-making and thus creative thought as a process of such geometric transformations (translations, rotations, reductions, reflections, etc.) of conceptual objects. Clearly though I have neither the math nor computer skills to attempt to execute such modeling. Now though to return to your very interesting comment about the physiologic response and describing the geometry of color along dimensions so defined. You move above the xkcd territory here! And into the fact that the color information from the three color receptors get segregated into two streams in the primary visual cortex: the L-M neurons that handle red-green opponent processing (overlapping greatly both differentially in their wavelength responses); and S/(L+M) neurons that handle blue-yellow opponent processing. But not _sure_ if we can describe hue in terms of activation of each channel as color perception is also subject to higher order processing and to input from higher order centers priming or inhibiting responses downstream. And while it is hard to think of representations in more than three dimensions clearly the math to manipulate objets of greater dimensionality exists. Lawd knows I don’t have the chops for it though! Thank you for the very fascinating comments! [Previous page](https://boards.straightdope.com/t/why-three-primary-colors-and-why-red-yellow-and-blue/800583.md?page=1)