Brown was always the first choice and still is at top level showing however i have shown sucessfully for the last 20 odd yrs with black tack and have only this last summer gone back to brown as that was the colour saddle that fitted - LOL
Brown tack looks naff to me on a black pony/horse so i never show my black one is brown tack but my others are shown in either dependant what class it is.
However, some black horses just look flat out pants in brown tack in which case black tack is used and is fine.... I think the Humdinger is shown in black tack who is owned by FMM on here! and they do HOYs etc etc (and do incredibly well!)
also some greys are shown at least to county level in black tack, again just because it makes the horse look smarter as some just dont suit brown!
However, some super anal judges may mark you down for black tack on say.... a bay horse, or a dark bay horse......... but some people would say they are right to do so!