what kelnor says is true, the salamanders are primarily a black space marine chapter.
just because there doesnt seem to be black people doesnt mean there aren't any. in the horus heresy books mersadie oliton is (i think) black.
the only reaqson that most models people paint are white is because it is much easier to do with the paints provided by the games workshop, do you know how hard it is to make black skin look good??