Yes, right from the start, it's said that he has a deep permanent tan from all the times he has spent in deserts and tropics. This seems kinda doubtful from a medical angle, you'd think the deepest tan would fade eventually unless his skin was really damaged.
Some fans (including myself) speculate that Doc's mother was a Mayan princess from the lost tribe in the Valley of the Vanished. This was the source of Doc's gold that was the basis of his fortune. I like this theory, Doc would then be related to the Mayan royalty; he protected and helped the tribe over the years and they supported his work.
A few think Doc was supposed to be black (that is, African-American). I doubt it, there's nothing to support this other than the dark skin and Doc is several times described as "the first white man" to enter a village in Tibet or journey up a South American river.