system
Given the following post, respond as if you're commenting on Reddit:
human
[Sci-Fi] How are there aliens that look exactly like humans in other galaxies, apart from their skin color? Also, how did they evolve to have blue/green/purple skin?

I understand that on Earth people tend to have darker brown skin if they're from a sunnier part of the planet, but what would possibly make them evolve to have green skin?
gpt
>How are there aliens that look exactly like humans in other galaxies

Barring handwaves such as a common ancestor (near-humans of *Star Wars*) or essentially-magical common influence (*Doctor Who*), it is generally asserted that bilaterally symmetric quadruped bipedality is highly evolutionarily advantageous, at least in a mostly-similar environment (therefore capable of walking in the same corridors as human charecters), resulting in convergent evolution, similar to how the eye emerged independently evolved in multiple lineages on Earth.
>Also, how did they evolve to have blue/green/purple skin?

Because it was evolutionary advantageous to have whatever skin pigment they have, with additional caveats for whatever biochemistry they have. Why plants are green? Chlorophyll is hardly the only option; it may be possible to have bright white titanium oxide pigments, or biological solar panels composed of germanium. And an alien with coboglobin would blush blue because the oxygen carrier in the blood is blue and not red, and their traffic lights would probably reflect association of spilled blood and violence with a wholly different color.