Tomahawkers don't have white skin, their bodies are painted to show their status. Tomahawkers are "upgraded" from green mudokons. Abe does that using shrines. I guess the way Toms use colors is part of their culture.
A green mudokon being upgraded:
Same picture, larger version
Another green mudokon being upgraded, with a Tomahawker in detail:
Same picture, larger version