In Greg Benford’s Foundation’s Fear there’s mention of a planet in the days before the fall of the Galactic Empire where humans are green. I don’t know the hows or whys–Benford doesn’t explain. It seems to me that it would be an adaptation to their environment, and a green pigmentation provided a protection against unusual environmental conditions.
The one issue with 24th-century adaptations occurring is that the environments we’ve seen are either extremely similar to Earth norms, or they can be tweaked to match Earth norms. We’ve seen no high-gravity babies, or humans adapted for zero-gee environments, or high ultraviolet, or low-oxygen atmospheres. When it’s possible to generate artificial gravity, when you can filter the atmosphere, or erect a radiation shield, the adaptive process is short-circuited.
I should step back for a moment and do something I don’t ordinarily do, and acknowledge that the reason we don’t see these things is because Star Trek is a television show, and the actors essaying the various roles were designed by genes and environment to look as though they came about in an environment just like Earth’s, so in film Trek, short of make-up or f/x, it’s impossible to have humans ten feet tall or albinic, or with extra-large chests because their lungs are far larger than normal because the air they’re used to breathing has far less oxygen than we are accustomed to. Print Trek doesn’t have this limitation, but it’s not a well often gone to.