system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't people have different natural hair colors like blue or green?

Are there any natural hair colors besides red, black, brown, and blonde?
gpt
Mammals main source of coloring is melanin, which produces browns, blacks and shade of tan.  Other animals such as birds have multiple types of pigment along with melanin including, carotenoids which produce red, orange or yellow feathers.  Also Porphyrins, an amino acid that can produce red, brown, pink and green colors.  The source of these pigments comes from many of the foods that birds eat.  So the way a birds body digests and converts these substances is another thing that our bodies did not evolve to do.  The forth material that produces colors like blue and green is keratin.  Our hair is made up of this same material although the way keratin is used in other animals is a bit different.  The structure of the keratin reflects certain light waves which makes it appear blue. The color of the keratin isn't blue itself. A birds feathers on a microscopic level can have a structure that reflects blue light.  Butterfly wings also work in this way.  Our hair doesn't have this complex structure to do the same.