Eye color depends on the amount of melanin - the pigment, which also determines the color of skin and hair. The more melanin, the darker the human eye. Accordingly, most dark-eyed people on the ground - Nilotic (a group of people in Central and East Africa). However, in Africa, there are blue-eyed people. It is albino. These organisms melanin very little, and the color of their eyes - "colorless yellow" - same as in albino all other continents.
It is believed that blindness - especially men's 'participation'. Indeed, they are in one way or another affect approximately 8% of men and only 1% of women. But it is also true that women tend to remember a lot more color names. Agree to distinguish from cyan indigo much easier when you imagine that all the words mean and what colors they stand. So men are just too lazy to memorize the names of flowers.
We are all born with conventionally light-gray eyes, and their "real" color appears only in year-three. Thus it can change during the life, such as during pregnancy or after injury. Most blue-eyed Europeans - the inhabitants of the Baltic States, northern Poland, Finland and Sweden. Most dark-eyed - Southerners (from Turkey to Portugal). By the way, the eyes of the most rare colors - green - can boast of only 2% of the world's population.
In theory, the human eye can distinguish 10 million colors and about 500 shades of gray. However, in practice, it is considered good ability to distinguish at least 150 colors (and after long training)