People are way too caught up on the color of Kerbals. Just look at the diversity in color of mammals on Earth and it's easy to see how selection, both natural and engineered, can account for their greeness. Maybe ancient Kerbin was dense jungle and it's effective camoflauge. Maybe the sun's rays hit Kerbin differently and the green helps diffuse harmful rays. Maybe green is sexy to Kerbals and they naturally evolved it through breeding. Don't know, but there are lots of potential reasons.
It's their shape that is the head-scratcher. That giant, flat-topped head. Short arms. Insane center of balance...
It's the why they have those bodies that troubles me.