Pink is not a colour

Have a look at the visible spectrum; you can see many colours: red, blue, green, yellow... but what about pink?

You can not find it in the spectrum... but then, why do we see it? Basically, we could say that the reason is just the difference between wavelength (property of waves) and colour (asigned by the brain).

When the eye perceives just one wavelength (for example 600 nm), our brain identifies the colour of that wavelength (in this example, red).

But, what happens if the eye receives light of more than one wavelength? In this case, the colour interpreted by the brain is usually the sum of the input responses on the retina, i.e. the colour halfway between them... except when the wavelengths come from both ends of the light spectrum at once (i.e. red and violet light).

In this case, the colour halfway would be green (not very representative of the mixture), so the brain simply invents a new colour halfway between them: pink (or magenta, according to its official name).

This post has been adapted from http://www.biotele.com/magenta.html. You can find there a extended version.