system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are human eyes usually blue, brown or green as opposed to any other colors?
gpt
The stroma (top layer of the iris) is what determines eye color. For those with no melanin (pigment that makes eyes or skin a warm brown), the only color comes from the Tyndall effect of light scattering in the iris. This creates a blue color, like water with glacial flour or dirty smoke.

When the stroma has a little melanin, it looks green because this blue combines with the the orangey brownish melanin. When it has a lot of melanin, the brown color takes over completely.