Cones are the photoreceptor partners to the rod cells in your retina. There are three types which respond to different wavelengths of light and allow you to see colour. They work a bit like the pixels on your screen in reverse – magenta, as in the small drawing here, is a mix of cyan and red. The cones sensitive to these blue and red ends of the spectrum will both respond and we perceive a purple. Similarly, I could have drawn a red and a green cone responding to yellow. The reason I didn’t is that about 1 in 12 of you, mainly males, would have had trouble distinguishing them due to colour vision deficiency, commonly called red-green colour blindness. This results from changes in the sensitivity of certain classes of cones.
How colour vision actually works is enormously complicated and the subject of ongoing debate. Cones do not respondly neatly to simply red, green or blue. Colour processing is not exclusive to the retina, several subsequent brain regions are involved as the signals travel to the visual cortex. Our perception of colour is influenced by surrounding colours and the brightness (luminance) of the image. This is perhaps not surprising when you consider the cenral role colour vision plays in human perception, whether this is the beauties of the natural world or the works of great artists…
As a final thought, there are reports of people who can see into the ultraviolet region and others who have a fourth type of cone. For birds this is a normal ability and gives them access to a vastly extended range of colours that defy our comprehension. This is described brilliantly in one of my all-time favourite popular science books ‘An Immense World’ by Ed Yong.