So which are the primary colors?

This node aims to clarify what is meant by primary colours, and how the two sets are related. At the end, it addresses why those colours are the primaries.

First, the practical answer to the question, they both are, but it depends on whether you are talking about light emitted from a source (TV tubes), or light reflected from a surface (paint).

What conventional physics teaches us

In the first case--emitted light--the colours are additive. As you add more colours, the hue gets lighter and brighter until you eventually get an off-white colour, or (if you do it exactly right) pure white.

In the second case, it is subtractive. As you add more and more paints to a canvas, the hue gets darker and darker and eventually ends up as a muddy brown, or possibly black, if you do it exactly right.

The first step in understanding all this is to go back to that boring old physics class when they told you about visible light being made up of a spectrum of colours : Richard of York Gave Battle In Vain or something. This mnemonic signified the colours of the rainbow:

They showed you this diagram. It is vital for any clear understanding of colour theory to get a clear picture of what is going on here.

```

___,---___
__/          \__
/                \
/       RED        \
/                    \
_|,---___      __ --- |
__/ |       \____/       |\__
/    |         /\         |   \
/     \ YELLOW /  \MAGENTA /    \
/       \      /    \      /      \
|        \__   |WHITE   __/       |
|           \__|    |__/          |
|              |---/|             |
\    GREEN     \CYAN/     BLUE    /
\              \  /             /
\__           _\/__         __/
\__     __/     \__   __/
---,           --,

```

If you take three spotlights, red, blue and green and shine them on a white card, the beams will merge and become white where all three overlap. Where red and blue overlap, you get magenta. Green and red gives yellow, while green and blue together make cyan. The point about the primaries (red, green blue) is that they cannot be made by mixing the other colours.

So we can say that to trigger the sensation of white in our brains, we need a dollop of red, a dollop of green and a dollop of blue.

Mixing paints

Now comes the tricky bit. When mixing paints, we all know that when you mix yellow and blue you get green. That pretty art class teacher told us that the primary colours were blue red and yellow, and she was much cuter than the physics teacher, so we believed her rather than him.

Thing is, he was right, and she was only half right.

Here is how paints work. They give your eye the sensation of yellow like this

```
White =RGB     RG (what the observer sees)
\\\   //
\\\ //
\\X/    Yellow surface
==============\X/===============
|
B (absorbed)
```

Remember that white light is made up of RGB colours. So imagine a beam of RGB falling onto a yellow surface. The yellow surface absorbs blue light. All the blue light on the surface gets soaked up, while all the red and green light gets reflected right back. Looking back up at the main diagram, R+G=Yellow. So our eye perceives the paint as yellow, because the paint has subtracted all the blue from white light.

Now think about a magenta paint.

```

White =RGB     RB (what the observer sees)
\\\   //
\\\ //
\\X/      Magenta surface
==============\X/===============
|
G (absorbed)
```

This time it is the green which is absorbed, leaving the red and blue colours to combine to give the observer the impression of magenta. Again, white light, with the green filtered out leaves magenta.

If you mix these two paints together, then the diagram looks a bit like this;

```
White =RGB       R (what the observer sees)
\\\     /
\\\   /
\\\ /  Mixed surface
===============\\/==============
||
BG (absorbed)

```

So when combining magenta and yellow, the resulting paint looks red because the blue and green parts of the spectrum have been absorbed--taken away from the white light.

Here is the full subtractive diagram (as they should teach it in art school)

```

___,---,__
__/          \__
/                \
/       CYAN       \
/                    \
_|,---.__      __,----|
__/ |       \____/       |\__
/    |         /\         |   \
/     \  BLUE  /  \ GREEN  /    \
/       \      /    \      /      \
|        \__   BLACK|   __/       |
|           \__|    |__/          |
|              |---/|             |
\    MAGENTA   \RED /   YELLOW    /
\              \  /             /
\__           _\/__         __/
\__     __/     \__   __/
'---'           --'

```

So this diagram says that if you mix cyan and magenta paints, you get blue. In the end that art school teacher was wrong, because she said the primary colours are red blue and yellow when she should have said magenta, cyan and yellow.

Printers have known this for years, and they use inks in the CMYK colours to achieve bright, photographic reproduction, but somehow, it seems all those art school teacher classes still get it wrong. They all claim that the primary colours are red blue and yellow, which explains why their students end up with muddy colours rather than bright vivid ones.

Try it next time you get out the paints. Use cyan and magenta as your primaries instead of blue and red, You'll be amazed at the difference.

Update26 Nov, 2004 I've wanted to do this for a long time, but here's a new section.

#### Why Red Green, Blue?

It's the receptors in your retina that make the red green and blue colours special. Nothing to do with physics or frequencies. Think about it, The cone-shaped light sensors in your retina are sensitive to red, green and blue light--or so says the theory, but apparently, their peak sensitivities are closer to yellow, green, and violet. Different amounts of stimulation on those receptors make for different sensations of colour in the brain.

If some of those receptors are sensitive to one particular frequency, in a narrow band filter kind of way, then that will be a primary colour. You won't be able to mimic it by using other frequencies. So where the cone-shaped light receptors in your retina respond to this to this or that frequency of light, those define the primary colours for each individual.

So far as physics is concerned, there is one continuous electromagnetic spectrum, with no special status afforded to any frequency. It took me a while to realise that the reason green is a primary colour and orange is not, is simply that we have receptors in our eyes which respond only to the frequency that corresponds to green light. This is why some people can detect four primary colours. The ability to sense colours depends on our individual biology: not on any external physical principles.

For more on this, see Golem's excellent writeup under retina

Further reading: CMYK, CcMmYK, RGB, colour theory, color theory, colour, color, Munsell system, Pantone system, secondary colors,