system
Given the following post, respond as if you're commenting on Reddit:
human
Why aren't there more eye colors?

Why are human eye colors so limited (when compared to the rainbow spectrum). Have we just selected for those colors or does it have to do with the actual composition of the eye?
gpt
Eye color pigments only vary from light brown to black. No green or blue pigments contribute to eye color. The variations you see in blue, green and hazel eyes are produced through not only a lack of dark pigmentation in the stroma but also the presence of Tyndall scattering, Rayleigh scattering and selective light absorption of certain biological molecules like hemoglobin - all this results in blue light being reflected more than red light. Kind of a cool optical trick. 

With blue eyes, the iris is pigmented and the stroma lacks pigmentation. Green eyes result from similar pigmentation in the iris but some light brown pigments in the stroma. Hazel eyes result from significant brown pigmentation in the iris’ anterior border layer and light brown pigments in the stroma. Brown eyes contain larger amounts of pigmentation (melanin) in the stroma. A lack of pigmentation in both the stroma and iris, Albinism, produces red or violet eye colors. In summation, eye color is limited because humans only have brown pigmentation. Diversity results from the brown pigmentation range (amber to black), various distributions of pigments between the stroma and iris and Tyndall scattering in the stroma.

Eye color is a polygenic trait. At least six genes are known to contribute to eye color phenotypes through at least 41 single-nucleotide polymorphisms (SNPs). However, the model is incomplete, because it is not yet 100% predictive.