They can look however you want them to, and I'd say there's a fair justification of whatever you settle on, really. In the movies, they are largely of light-toned skin with darker hair, based on the glimpse of the Easterlings we get in The Two Towers and the guy that Faramir shoots down from the Mumak. The Mahoud chieftans controlling the Mumak in the movie look like caucasians to me. So the movies (which the game's aesthetic is based on, after all) depict them to be of only slightly darker skin tones.

Conversely, in the books I do get the impression that they were somewhat darker toned, with Haradrim close to middle easterners and Easterling more along the lines of the Mongolians. If you plan on getting any, I would probably paint the Mahud warriors with much darker skin-tones, since they look more like tribal Africans to me.