The eye color that a human being has is determined by genetics, but no one gene that ensures what that color will be. The OCA2 gene often controls the amount of melanin pigment that a body generates, which affects 74 percent of human eye color variation, but even that gene is impacted by many other factors in the DNA. Most human eyes are brown or blue, but other colors like hazel and gray are more rare. Also, different populations tend to have different eye colors that are more prevalent or rare.
Green Eye Color
Green eye color is the most common of the rare colors, as it is found throughout the world but is only in about 2 percent of the total world population. This color is produced by a large amount of melanin in the iris during development, which is a result of genetics. Like other rare eye colors, green eyes are often weeded out of a family because dominant traits like brown eyes are expressed more often.
Amber Eye Color
Amber-colored eyes are often called “wolf eyes” because they resemble the pigment of those in wolves. This eye color consists of a golden or yellow pigment with a bit of copper and rust tint. Lipochrome, the name of the yellow pigment, is extremely rare and as a result few people have this eye color.
Red Eye Color
Most often, red-colored eyes are found in humans who have pigment problems, such as albinos. Some rare cases exist in which people without pigment troubles will have red eyes; this pigment may be the result of a small leak of blood into the iris causing a reddish eye.
Violet Eye Color
There are a couple different situations that lead to eyes appearing violet. When there is not enough pigment in the iris to cover blood vessels, the light that reflects off these vessels make the eyes appear violet. This can also occur if a person with blue eyes has reddish bloodshot eyes from irritation, making the eyes appear violet. If this second situation is the case, the violet color in the eyes is temporary.
Black Eye Color
Some people believe that black eyes do not actually exist and that the color is really just an extremely dark brown. There have been a few cases, however, that point to purely black eyes in human beings according to the book "Evolution of the Eye."