The core problem for Mag’har Orcs being Warlocks is that the foundation for their race is based on not being corrupted by fel. The best and easiest solution is to not have Warlock as an available class to this race. Since I have doubts the best option will be implemented, let me propose an alternative in building the lore for supporting it.
Firstly, this combination should be locked behind a quest that explains how an uncorrupted Mag’har could be a Warlock. This would ensure the player has a coherent understanding of why this unthinkable combination could be allowed.
The unlock quest should contain some background that allows the Mag’har to be shielded from fel magic. Possibly a totem of some sort that absorbed the fel away from them? This would keep the Mag’har Orc from turning green.
I’m open to any type of background, the totem just seemed like a good premise.