Personally, every scrap of AR-15 furniture I own is black, except that original Colt 601 pistol grip I have somewhere which is the early grass green colour.
I applaud the fun colours that you see sometimes from Cavalry Arms and others. DeWalt Yellow, My Little Pony pink, stuff like that. I thinks it emphasizes that some of these guns are strictly fun and makes them more approachable for new shooters, especially women and kids.