One thing to keep in mind for orc roleplayers is that to Azeroth orcs’ knowledge, all the orcs living on Outland might as well be dead, and the Dark Portal closed forever.
Durotan and Draka are dismayed to find that Thrall is born with green skin, so older orcs probably know, and it’s written here and there that the green skin is a mark of shame that they bear.
From Chronicles volume 2, page 128:
“This should have been a beautiful moment for Draka and Durotan. Instead, it filled them with horror. Their son’s skin was green. He was infected with the orcs’ blood-curse. For Durotan, this was the last straw. Gul’dan’s pact with his masters had damned the orc race for all generations.”
And then they tell Orgrim about it, so I imagine the green skin = fel magic connection was probably known, but like many things considered lost history among some of the internment camp orcs.
This tidbit comes up when Garrosh comes to Orgrimmar as “the first Mag’har” - from Heart of War, page 4:
“That is ridiculous. You never even fought in the wars! You said you were children in the internment camps! Isn’t that enough punishment? Why should you suffer any more?”
“I bear the mark all the same,” she said, holding up her hands—green, as were her sister’s, as were all the orcs in Orgrimmar, save him. “I reap what all of them have sown. Is there not some payment owed?”
So there seems to be at least some cultural awareness - even from some of the internment camp orcs - that orcs weren’t always green.
Maybe it’s one of those things that younger generation orcs thought were just old people’s ramblings. If you’re raised as a gladiator slave, orc lore of coming from a different planet must seem like a very alien (hah!) concept.