Since most humans have three separate types of cones (making us trichromats), a total of (100)3 = 1 million colors are discernable to a typical human. Some humans are born without one of the three types of cones, creating a condition known as color blindness; color blind (dichromat) humans can only see (100)2 = 10,000 distinct colors. On the other hand, some humans have four distinct types of cones, making them tetrachromats and allowing them to distinguish up to (100)4 = 100 million separate colors!
http://scienceblogs.com/startswithab...-in-a-rainbow/