I try to make my force as diverse as possible, partially for realism and partially because I have more fun painting 7 squad sergeants in 7 different ways rather than have 7 models that look basically identical.
That said, any model that I attempted to paint as Asian, Hispanic, olive-toned, etc. all basically look like white guys who maybe got a bit more sun than their companions. The darker-skinned models either look like I just base-coated their faces brown without highlights, or if I used brighter highlights, they look like white guys with dirty faces. Painting dark skin is hard.