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.
However, from Tiefling Variants sidebar (Sword Coast Adventurer's Guide, p. 118):
Rather than having the physical characteristics described in the Player's Handbook, choose 1d4+1 following features: small horns; fangs or sharp teeth; a forked tongue; catlike eyes; six fingers on each hand; goatlike legs; cloven hoofs; a forked tail; leathery or scaly skin; red or dark blue skin; cast no shadow or reflection; exude a smell of brimstone.
So there is a rule here that allows tieflings to have blue skin. Unfortunately, this is the most I've seen in 5e material that refers to tieflings having blue skin. I have not been able to find any lore presented in 5e materials that might explain why a tiefling might have blue skin.
What lore exists to explain why a tiefling might have blue skin as opposed to the more usual red or "human skin colour"?
I've heard some people (online and IRL) say that it relates to the different bloodlines as per the options presented in Mordenkainen's Tome of Foes, but reading that section of that book, it doesn't mention skin colour at all, so I'm not sure where this interpretation came from but it doesn't appear to be RAW/official.
I'm looking for official lore, ideally from 5e but lore from previous editions is welcome too (I'm guessing AD&D 2e Planescape stuff might shed some light on this?).
I'm looking for lore from any official setting, but if that's not specific enough, let's say Forgotten Realms (the reason I'm reluctant to commit to a specific setting is because, as I understand it, all settings are technically part of the Planescape multiverse; someone can correct me in the comments if I've misinterpreted that).
- Is there a way to determine what physical traits your character could have based on their Diabolic bloodline? (closely related, but it asks "what appearances are for which fiendish bloodline", whereas I'm after any lore explanation for skin tone specifically)
- Can a tiefling have permanent ivory white skin? (loosely related, as this question was just asking to justify a skin tone choice without much emphasis on lore explanation)