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It is said that red, blue and yellow are the 3 primary colors, which can't be formed by any other color.

Then how come we create yellow color in RGB color model?

Is there any theory or reason behind this?

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    Even when talking about mixing paints, red, blue and yellow is a vast oversimplification (although there are some synthetic pigments that come close enough for primary school art classes). We generally use a scarlet and a crimson for "red", a lemon and something more like a gamboge for "yellow", and several blues (ultramarine, cobalt, cerulean and phthalo or Prussian), depending on the mixed colour we are trying to achieve. CMYK covers most of the mixtures fairly well with a smaller number of pigments. – Stan Rogers Aug 13 '15 at 16:21
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    You're confusing additive and subtractive color spaces. BRY (or CMY) are the primary subtractive colors. RGB are the primary additive colors. – DA01 Aug 13 '15 at 17:15
  • Not an exact duplicate, but the same answers apply: graphicdesign.stackexchange.com/questions/55016/… – DA01 Aug 13 '15 at 17:23
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Yes the human color vision is based on 3 color sensors. If we simplify things to bare basics then it goes like this:

RGB colors

Our brain then processes this information and simply shows a mixture of red and green as yellow. Likewise red and blue as magenta*. Ok so that explains the RGB colors.

Color sensors

The CMY colors

Or what is usually taught out as Red, Blue & Yellow is actually the inverse of RGB. See when light hits the paper you actually see the photons that the paper did not absorb. So Cyan is just a color that eats up all red colors, so absence of red.

This is called a subtractive color model. In reality there's no such thing, just that its easier to conceptually think when the color is reflective. All color that you see is from a additive color just the difference is where the light comes form the medium or another source.

In reality is much more complicated.

Image 1: human color receptors. Image courtesy to Wikipedia

* Magenta color does not actually exist, in the spectrum. There is no magenta color in light its just the mixture that our brain shows it as magenta.

  • It is too complex to understand. But after reading your answer several times and referring google I understand! and I also came to know that we primary colour varies and they are not fixed to red, blue and yellow. Thanks for the detailed answer. – bharat Aug 13 '15 at 12:50
  • @joojaa I'm confused by the statement 'magenta doesn't actually exist'. (It seems the argument could apply to all colors...color doesn't actually exist...it's all just what our brain interprets) – DA01 Aug 13 '15 at 20:10
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    It does not exist in the spectrum. @DA01 so theres no magenta color in a rainbow or a series of wavelengths. The color in outr brain exists but you need 2 colors of the spectra to make it. – joojaa Aug 13 '15 at 20:34
  • @joojaa oh! I get what you're saying. – DA01 Aug 13 '15 at 20:42
  • Please consider upvoting/following the StackExchange Color Theory site: area51.stackexchange.com/proposals/110687/color-theory – Adi Shavit Jun 22 '17 at 8:17
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It is said that Red, Blue & Yellow are the 3 primary colours

Yes it is said but it is wrong. The Red Blue Yellow is an unacurate historical model.

The 3 primary light colors are Red Blue and Green.

When you use a paper or a canvas you can not emit light, so you use the complementary color model that is the subtractive model, so the secondary colors become the primary pigments. Cyan, Magenta, and Yellow.

I'm making this image darker simulating a print, becouse it is harder to achive bright colors on a print:

Both color models are in fact the same. One 3D representation is a cube, that contains all colors. But you can rotate the cube and use it as a basis for this 2D circles I showed.

Here is my explanation on why the RYB model is inacurate.

Why is the opposite of yellow on color wheels sometimes purple and sometimes blue?

Yes you will see that it has some downvotes... That is interesting.

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    The downvotes on that other answer had more to do with the context of the question. It's important for people to understand the difference between subtractive and additive. You are correct that in the subtractive model, BRY isn't entirely accurate. That's one issue. The other, often bigger issue, is that people don't always understand additive vs. subtractive. – DA01 Aug 13 '15 at 17:18
  • In other words, the color models, in context, are not the same. (Philosophically, it's debatable, though) – DA01 Aug 13 '15 at 17:20
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    It's worth noting that when mixing two colors of light, the perceived color of the mix will generally be as predicted by the additive color model even if the original colors were not formed by mixing red, green, and blue, but the subtractive model will only predict the result of mixing pigments if those pigments are comprised of the same cyan, magenta, and yellow base pigments. Shining a mix of red and green light through a magenta filter will leave red, but shining sodium-flare yellow through a magenta filter will yield, if anything, a bright or dim yellow. – supercat Aug 13 '15 at 19:05
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Red, Blue, and Yellow are used for pigment when you print or mix colours. Actually, technically, we use CMYK, Cyan, Magenta, Yellow, and Black, but that's besides the point.

Light itself is actually made up from Red, Green, and Blue. Since you're using a computer the screen uses, essentially, red, green, and blue lights to form the colours you see. Hope that helps.

  • > Actually, technically, we use CMYK, Cyan, Magenta, Yellow, and Black, but that's besides the point. - Actually... that is the point. :o) – Rafael Aug 13 '15 at 15:54
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The simple answer:

When mixing pigments, you are using the subtractive color model. Blue, Red and Yellow are the primary colors in that color space.

When mixing light, you are using the additive color model. Red, Green and Yellow are the primary colors in that color space.

The accuracy of the "Blue, Red and Yellow" primary colors in the subtractive color space is definitely debatable. Most would agree it's not that accurate. But for the sake of this question, the answer is "Because the primary colors are different in each color space."

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    Also debatable if you are refering to a color space or a color model :o) – Rafael Aug 13 '15 at 20:01
  • @Rafael yea, they're technically different things (space and model) though in this context, related pretty closely. – DA01 Aug 13 '15 at 20:07
  • But why the downvote? – DA01 Aug 13 '15 at 20:07
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    (Was not me)... :o) – Rafael Aug 13 '15 at 21:31
  • @Rafael we must find the mysterious downvoter! :) – DA01 Aug 13 '15 at 21:38

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