Atomic as a powerset is inspired by Firestorm; whichever villain is selected to be the "inspired by" character on the select screen is irrelevant. Like, I think Superman is the Ice hero. Means nothing. Atomic is based on Firestorm, not Atomic Skull, so it doesn't really matter what color he is. Besides, I know everyone thinks of green when they think of Atomic but that doesn't really make any sense. Stars create nuclear radiation but there are no green stars.