Here is my chestnut wearing his black gear.
And my grey...
However, I do know quite a few people who have brown on their chestnuts and I do think it looks very smart.
It's really down to personal preferance and I am sorry, for people to say "oh you will be kicked out of the barn if you show up with black tack," that just goes to show how narrow minded they are. If your tack fits well, is clean and well looked after, who cares what colour it is??