system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why are green eyes so rare?

They seem pretty low in percentage (2-3%) compared to other eye colours. Aren't blue eyes a mutation? Why aren't they nearly as rare?
gpt
When you make blue eyes you only need a light (lacking in melanin) top part (stroma) and a dark (containing melanin) bottom part (epithelium), so that some colors of light bounce all around in the top layer and look colorful (Rayleigh scattering in the stroma- same thing that makes the sky blue) while others "stick" to the under layer (are absorbed by the darker epithelium).

Green eyes require a more careful pairing of colors (more specific and greater concentration of melanin in the stroma) in order for some of the pretty brown light to bounce around with the pretty blue light instead of all just sticking to the bottom layer. It's like when you mix blue and yellow play dough, except with bits of light. If you look carefully (but not too close), some people's eyes look like they change color when you see them at a different angle; that's because of the different ways that the light bounces out of their eyes and into yours.