As a counterpoint: Saruman the White and his horde of orcs were about as evil as they come, there were evil humans of both light and dark skin, and that applies to many of the "good" races. That said, yes, the evil parties tended to be of dark complexion, but I think of that as less a racial undertone, and more likely a literal illustration of the classic good == light, evil == dark parallel (for instance, Sauron being called the "Dark Lord", many references to "the darkness", etc.). Who knows though, perhaps Tolkein was racist, I just never felt that his writings came across that way.