Brown. If you are showing, then traditionalists will say brown.
But there again, I have a piebald native, which I show in (horrors) a black dressage saddle.........
However - if my 'saddle of a lifetime at a price I was able to afford' had been brown, then that would have made it even better. Black does not always look as good as brown/havanna after a few years. It tends to fade a bit whereas havanna goes darker and richer.
Brown tack without a doubt. It looks better with age & gets a real lustre as it ages. Black eventually goes grey no matter how much you look after it.