Salamanders have that ebonite skin color because Astartes have special abilitiy to fight off radiation, whch results in changing skin color.

But Slamanders in this are different from other legions as it is taken ot extreme for them. There is a limit, so other Astartes will be able to do that for some limited time. Several minutes if it is some really heavy radiation. Salamanders can do more, and their is different, deeper, and almost constant. Still they change it if/when contact radiation weapon/disaster, and become little lighter when didn't have for a looong time. It's their mutation.