Rules? There are no rules.
I seem to prefer tan-walls on basically everything however. It's just more "pro." Black can look real nice on dark-coloured bikes, and ones with a lot of black components. If decent mountain bike tires came with tan walls I'd probably run those as well.
I still haven't seen a bike with coloured tires that would look better with tan walls or all-black tires. Maybe a black and chrome bike would look good with tires with a red stripe on each side, but I bet I'd still prefer my red accent to be somewhere else, like the cables or grips.