Tieflings (in 5e, at least) have either “human skin colour”, red skin, or somewhere in between. I think 4e changed how tieflings look between 2/3/3.5e and 5e, standardising the red skin/Asmodeus look, but I believe that the tiefling’s skin colour has always been within this range from the little I know of the previous editions.
From the 5e tiefling player race description, Infernal Bloodline section (PHB, p. 42):
Their skin tones cover the full range of human coloration, but also include various shades of red.
The official art I’ve seen of tieflings in 5e (and, again, the little I’ve seen of previous editions) backs this up, showing mostly red tieflings.