I know it is possible to get really bright colors over a black primer, but it's not possible for me. So I use white for brighter look, grey or dark brown when the figure is mainly earth tones, and black for anything else.
I was using black for pretty much everything until earlier this year when I was painting some old AD&D gnomes, and just couldn't get the clean, "cold" look I wanted. Stripped and switched to white primer and was much happier with the results.