Tetrachromacy
Colour vision based on four kinds of cone cell instead of the usual human three.
Also: four-colour vision
Many birds, fish, reptiles and insects have four or more kinds of colour-sensing cell, often including one tuned to ultraviolet. They see differences that are invisible to us; a plain-looking bird may be brightly patterned to its own kind. Most mammals have only two kinds, and humans and some other primates have three. A small number of women carry a fourth cone type. Research suggests that only a few of them can actually use it to see extra distinctions between colours, and the subject is still being studied.