Tetrachromacy is the ability to see light from four distinct sources. An example of this in the animal kingdom is the zebrafish (Danio rerio), which can see light from the red, green, blue, and ultraviolet sections of the light spectrum. True tetrachromacy in humans is much rarer however – according to Wikipedia only two possible tetrachromats have been identified.
Humans are normally trichromats, having three types of cone cells that receive light from either the red, green, or blue part of the light spectrum. Each cone can pick up about 100 graduations of color and the brain combines colors and graduations so that there are about 1 million distinguishable hues coloring your world. A true tetrachromat with an extra type of cone between red and green (in the orange range) would, theoretically, be able to perceive 100 million colors.
Like supertasting, tetrachromacy is thought to be much more common in women than men – estimates range from 2 – 3% to 50% of women. Interestingly, colour-blindness in men (much more common than in women) may be inherited from women with tetrachromacy.