Some time in the distant past, our ancestors moved to the treetops and decided that fruit was quite delicious. Before that, we weren’t fussy. Afterwards, fruit was top of the menu. And so we evolved three-colour vision to distinguish unripe fruit from the very ripest. Or, that’s one theory. In truth, we don’t know for sure what caused our colour vision to change in the deep past. But for whatever reason, although most mammals see the world in two colours, most humans see the world in three. It’s called trichromacy.
Now, I know what you’re thinking. There are more than three colours. Richard of York gained battle in vain for this. Well, hold your horses, because poor Richard did not gain battle in vain in vain. There are many, many colours, but, for most humans, these are all mixes of red, green, and blue. Trichromats have three colour receptors, called ‘cones’, at the back of their eyes, and all the colours of the rainbow are constructed from these three basic building blocks. Different ratios of red, green, and blue can create most any colour. And those of you with three-colour vision can try it out for yourself using the colour twiddler in Microsoft Paint.