I've seen the discussion on the forum before. Mars' skin tone changes by the scene. Sometimes he's solid black, sometimes grey looking, sometimes brown, sometimes white. Look in the gallery at the early scenes with Sera and then look at the newer ones. He looked white originally and he's moved to a much greyer color now. In her ending, he looks as dark or darker than her. It's not character specific either. He looked greyish and ash colored in some scenes with Elena and much darker brown in others. Maybe Nitro will give some sort of an explanation in the future, but so far I've only ever seen people here say he claims to still be using the same coloring. The coloring is definitely different, but I don't really know if it's an interracial thing or not. Maybe he's trying to make Mars seem different from everyone else or maybe the other guy's theory about corruption is right. I really doubt we'll ever hear a clear answer on it from Nitrolith unless it really is plot related.