Why Do People Have Different Eye Colors?

What Is Melanin?

Melanin is a name for a group of pigments found in most organisms, including us, humans! Melanin is what makes our skin and hair appear darker or take a specific shade. We also have melanin in our eyes, but we will get to it soon.

Melanin group includes several pigments, each "responsible" for their color. Those that impact our external appearance the most are brown eumelanin, black eumelanin, and yellow and red pigment pheomelanin. Eumelanin is both the most common and the most abundant, and typically is the one "missing" in cases of albinism. 

How Melanin Determines Our Appearance

You have seen the effects of the changes in melanin in human bodies many times! For example, eumelanin in the reason we get gray hair with aging. Our hair itself is more or less transparent, and it is melanins that give it color. As the body ages, it stops producing brown eumelanin but continues producing the black eumelanin; hence the hair turns grey. At an even later age, even black one can fade away, leaving hair with no color (it appears white because of the optical effects). 

The ocular lens is transparent, so, same as our skin, our eyes need it for protection from UV and other high-frequency light. This is why iris needs its pigments. 

Melanin + Optical Physics = Eye Color

The pigmented areas of the iris consist of several layers: the front fibrovascular layer known as stroma (contains pigmented elements of brown or yellow color) and, beneath the stroma, pigmented epithelial cells (mainly black). Iris' shade is a very complex phenomenon: each visible, percieved color is a result of the combined effects of pigmentation, texture, and interlacing of the tissues and blood vessels within the iris. 

Pigmentation is still the most impactful factor, so if there is a lot of melanin in the eye, they would appear dark-colored. If there are no pigments at all, the iris would appear pink (the color of the blood vessels) or purplish (blood vessels plus some light scattering). 

In the eyes with little melanin in the stroma, various optical phenomena produce shades of blue and green. Those processes are very complex: selective absorption and reflection by biological molecules, Rayleigh scattering, Tyndall scattering, and diffraction - to name a few.  

Eyes with light yellow or amber color happen when the pigmented stromal cells contain pheomelanin with little eumelanin. If you only see yellow on the strong pupil contraction (when you stare at the bright light), it might be just the color of the contracting fibers.