Several people draw her black. I find it interesting.
She always struck me as the type who would be Asian if she were human, and not just because of the intelligence thing. I don't think I've seen a picture of Twilight that was darker than 'mixed', except that one troll drawing of her after everyone complained about the person drawing her with skin darker than white. I have seen black Celestia, and I loved that picture so much that I can't think of her any other way.
It really depends.
I like Twilight to be dark skinned, and Fluttershy to be dressed modestly (not super cute). Those are the biggest things I like for them.
Every one else has a whole bunch of 'do-nots.'