system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 how come eyes don’t have more colors? Why is it just brown, blue, green, hazel?

How come we don’t have mostly purple, red, orange, etc. colored eyes?
gpt
Optometrist here.  Eye color is a function of pigment (which is brown).  All of us (unless you have albinism) have pigment on the back side of the iris.  Blue eyed people have little to no pigment on the front side of the iris.  It looks blue for the same reason the sky looks blue (essentially).  Light enters, goes though the iris, scatters and reflects off the pigment on the back.  

People with colors other than blue have varying amounts of pigment on the front surface of the iris.  The darker the eyes, the more pigment.  Nobody actually has true "green" eyes when viewed under a microscope, however, many people have colors that can appear greenish especially under some lighting conditions. Some eyes do have a bit of a grey appearance, as well.  

There is a lot of subtle variation between people.  I've seen people with some of the deepest, most gorgeous shades of blue you can imagine.  The "scientist" in me would explain it with a thicker iris creating more scatter, but the "artistic" side of me just wants it to be 'beautiful' just because it is. Contrary to what another poster said, eyes do not change color based on mood.  In fact, the eyes do not change color at all. However, the reflections and the scatter can vary depending on pupil size and lighting in the environment, so some eyes do appear to change color a little bit.