When I showed my paint gelding, I would use black because he had black feet. I was told it was more of a breed thing to use black (I did not do breed shows with him), but I felt like it gave the illusion of him looking a little more correct in front. Normally I use oil and not polish, but I think the black was the best choice for him.
I showed him in hunter in hand, color, and show grooming:
And his feet normally (he has black ermine spots, one of his front feet has a stripe)
And a most recent picture of him...he is almost 25 now.