Why Can Human Eyes Only Be Brown, Green, and Blue?

Other color combinations that can occur in the human eye are also completely related to the surprising answer to this question.

The structure that gives color to the eye is called the iris. The iris consists of two layers: the back layer containing melanin pigment and the thick front layer called the stroma. This front layer may also contain melanin pigment, but sometimes it doesn’t, or contains very little. When the layer called stroma contains a high amount of melanin pigment, our eyes appear brown because melanin is brown in color. When it contains no melanin pigment, our eyes appear blue. However, this is not because the pigment is blue; just like the sky appears blue, it looks this way due to excessive refraction and scattering of light in that layer.

This photo shows how the eye is colored in the presence and absence of brown pigment

For green eyes to form, instead of having very dense or no melanin pigment, it occurs as a result of having low density of melanin. The eye appears green when a honey-like color mixes into the blue.

So how do red eyes, seen in albino people?

Albino people have no melanin pigment at all; there is no pigment in either the front or back layer of the iris. This makes the iris extremely permeable to light. The large amount of light entering the eye hits the blood vessels in that person’s eye, causing the eye to appear red. This situation also causes the “red eye” effect when a camera flash goes off. The pigment density of the iris can change over time, so people who say “I had blue eyes when I was little” are not actually lying. They just had a small amount of melanin pigment in their eyes when they were young and their eyes appeared blue, but over time these pigments increased and their eyes began to appear brown. Also, people who claim that “their eyes are blue in winter and green in summer” are not lying either. The small amount of melanin pigment in their eyes becomes darker in summer, just like we get tanned when we go out in the sun in summer, causing their eyes to appear a different color.