I had no idea that Nords could be darker-skinned in Skyrim. I've found mods that make it possible, but I didn't know that it was possible in the vanilla game. That said, looking back at the lore (for all TES games), Redguards have always been the "dark-skinned" race for humans. As it's already been said, part of this has to do with the culture each race is based upon. Imperials would be the only ones that might be able to lean toward the darker-skinned (not including non-human, which seems to not be a choice for you), but I haven't really paid that much attention to what their culture is based upon...so...
Anyway, there is an image floating around the internet that compares the appearances of the races of Tamriel since the beginning of TES games. Redguards have been the only dark-skinned race for humans since the beginning. I'm not saying that it's "right" or anything, just saying that there's a certain continuity of lore. (Edit: I would have posted a link to this image, but cannot, so I'm sorry D:)