Originally Posted by
Vyve
Ehm, no. First of all, Blackhand is brown too, even though he is nothing but a bad guy. Second, the whole thing is actually logical given how the story unfolds in the Cinematic Universe.
1. The Frostwolves do not join the Horde until the opening of the Portal. As they do not have warlocks (Drek'Thar stays a shaman), they are not exposed to fel magic. Hence their skin is brown. (Source: Warcraft: Durotan movie prequel book).
2. Thrall is corrupted when Draka crosses to Azeroth. The fel energies of the Portal basically affects him in her womb.
3. The Frostwolves do not turn green because there is no time for them to do so. They die soon enough.
4. Blackhand is brown because he is is distrustful of the fel and shields away from it.