Question: Why are red, blue and yellow the primary colours? Also when I studied eyes it said the primary colours were red, blue and green. Why? What are the REAL primary colours?

Susan Cartwright answered on 18 Jun 2015:

This is the difference between additive and subtractive primary colours. If you are using light, then you are adding light of different colours, and – for example – red light and green light together stimulate your eye to perceive yellow. Because we have (only) three different colour receptors in our retinas, we only need three primary colours of light to simulate any other colour. (There is a “real” yellow – light at about 570 nm is yellow, without being a mixture of red and green. It stimulates both our “red” receptors and our “green” receptors, so it can be simulated by mixing red and green light. Sodium street-lamps – the dark orange-yellow ones – emit light at a wavelength of about 589 nm: they are “real” orange-yellow, not a mixture of red and green with more red than green, but your eye can’t tell the difference.)

Coloured paints work differently: there, the pigment absorbs all the other colours and reflects “its” colour – so red paint absorbs blue, green and yellow light and reflects red. If you add red and green paint, you are absorbing all the colours – in principle you should get black; in practice, because the absorption is not complete, you get a muddy brown. So the subtractive primary colours are different from the additive ones. In principle, the best subtractive primary colours are cyan (blue+green), magenta (blue+red) and yellow – they’re the opposites of the additive colours. In practice, red can substitute for magenta, and blue for cyan, giving you the paintbox primary colours of red, blue and yellow.

If we had more than 3 types of receptor, we would need more than 3 primary colours – for example, if we had a “yellow” receptor, a mixture of red and green would probably NOT look like yellow (it would stimulate the red and the green receptors, and probably “leak” into the yellow receptor, but the result would be “lots of red” + “some yellow” + “lots of green”, whereas real yellow would be “some red” + “lots of yellow” + “some green”, which would be different). Because red and green colour vision comes from genes on the X chromosome, some women actually have 4 different receptors (because they have two different genes which code for slightly different wavelength ranges). They see a richer variety of colours, but because the different receptors are only a bit different, they don’t need 4 different primary colours – they find 3-colour systems like TVs and computer monitors satisfactory, whereas someone with a yellow receptor would not.

Some animals have very different colour vision from us, and would require different sets of primary colours – for example, many insects and some birds can see into the ultraviolet. The mantis shrimp has no less than 12 types of receptor!! It would need 12 primary colours, in principle. In fact, it appears that instead of “mixing” the responses from its 12 different receptors (so that red+green=yellow) it appears that the mantis shrimp simply picks the one with the biggest signal, so its colour vision isn’t any more sensitive than ours – but it requires a lot less processing in the brain, so it’s better for the shrimp than our type of colour vision would be.