Asked by: Pavel Sizov, London
Usually because they have more or fewer types of cone cells, the wavelength sensitive photoreceptors in the retina at the back of their eyes. Most people are trichromats, having three types of cone. Dichromats have one of the cone types missing. Men are far more prone to dichromacy than women because the genes involved are on the X chromosome, so the trait is sex-linked. Rarer still are monochromats who have two or even all three of the cone pigments missing. This is known as total colour blindness.
Experiments can easily show which colours someone can distinguish, but it is not so easy to know how the colours look to them. This is especially peculiar when it comes to the rare cases of tetrachromacy. These are people, mostly women, who have an extra set of cones. They can distinguish far more colours than anyone else. But what are those colours, and could the rest of us ever know what their world looks like?