The appearance of color in a person's eye is a function of the pigments included within the iris. Specific colors are determined by the individual's genes, making some eye colors more common than others.
The most common eye color throughout the world is brown. A high amount of melanin, the pigment that also causes darker hair and skin tone, will cause brown eye color. Very dark eyes may appear to be black. Brown eyes are common through all races and in all parts of the world.
Less common eye colors include blue (found among people of European descent with lower levels of melanin), hazel (a combination of green and brown), gray (a variant of blue mixed with other hues) and green (typically consolidated to people of Nordic origin). The rarest among these colors is green with only one to two percent of all people born with natural green eyes.
The rarest eye colors include amber, violet and red. Amber is the result of the yellow pigment lipochrome. Violet eyes are believed to be the result of a lack of enough pigment to fill the entire eye, allowing the blood vessels to become visible. Red, likely the rarest of all human eye colors, is the result of albinism, where the eye lacks any pigment.