The title is just a fancy way of saying that some women seem to have four colour receptors in their eyes rather than the usual three. Actually some people also have a different set of three and this was known for some time before the discovery that quite a few women see extra colours than the rest of the population. Of course some men are missing a receptor and have only two and as a result are called colour blind. Compared to tetrachromats we are all colour blind. Join discussion...