50% of women contain a 4th color receptor in their eyes in addition to the normal 3, and can thus distinguish between more colors
As an example of how this works, where most of us may look at a rainbow and see seven colors, a lucky lady with that fourth receptor will actually be able to make out 10. An even smaller number of women end up with 5 color receptors!
No wonder these ladies know a whole range of colours! Colours like Maroon and mauve and Ox blood and all that silly stuff!