Answer:
"It is both. The word "buck" or "buff" is thought to be from the color of a buckskin, which is the pale tan color of European skin - this gives you "buck naked" and "in the buff." "Butt naked" refers to the fact that your buttocks are not covered."
I have always heard it "buck naked". I don't think it refers to any color at all; the phrase was originally meant to compare one to an "Indian buck" as the men were called many years ago in a less politically correct age. They were commonly thought to be "naked savages", whether they were in fact or not. Thus, "buck naked" implied being without clothing.