Chaos orcs on Kalimdor drank the blood of Mannoroth the second time and turned all red. Fel Orcs in Outland, however, are both exposed to intense amounts of Fel Corruption, Nether energies and were drinking demon blood in multiple occasions to increase their raw brute strength. I believe it was even stated somewhere among the quests relating to Fel Orcs in TBC about their origins. Kargath Bladefist is among those to blame for their continued usage of demonic powers/blood I believe, as well as Illidan who used them as pawns.
Overall, Chaos and Fel orcs are not the same lore-wise, of course after Blizzard evolved it in such a way. As for Warlords of Draenor ones, well, I suppose Blizzard went for cool looks there and they surely did a great job with all that greyish-fel combo appearance, but in my honest opinion that change of appearance after one sip of demon blood was a bit over the top.