I have some e30 bottlecaps and I can get them done any colour at work cause I do powdercoating
I have seen a couple pictures of black and they arent as cool as I imagined :/ so maybe a dark silver/ grey?
I usually sandblast the older wheels and them powdercoat them in white or black, then paint them over in paintbooth professionally so they last really long and you have a wide range of colours available.
And I guess I know what i'm doing -> www.oldtimer-wheels.com <- this is my site, on the sold section are a lot of galleries with pictures of the refurb.