I learned the hard way once that even a (seemingly) innocent comment about a friend's tan could be perceived very adversely, due to the fact that their culture had "being tanned" == "being dark(er)" connotations which were quite negative. It's a particular point of view, not one globally shared, that being tanned (or general malleability of skin tone) is a discussion point/something of casual interest.
These day I find it's best to simply mind my own business about others' appearances unless they're very close friends or family.