I use brown dressage tack on my pony. I don't have any current pictures of it, but I might get one today when I ride if anyone is interested to see. She's a red roan with flaxen mane and tail, and I think that she looks much better in brown tack... but that's a personal choice. Her bridle isn't technically a "dressage" bridle (you can actually see it in my avvie picture to the left), but it doesn't really need to be until we get to the upper levels with a weymouth or something.
I think most horses look great in black dressage tack, and it can be more challenging to find brown dressage tack that all matches. There's different definitions of 'brown'... all sorts of shades and whatnot. Usually when you order something in black, you can be sure that it will be BLACK, not grey.
On an unrelated note... can anyone say 'piano hands' on that pic JDI posted of Lauren Collett?? I hope they just caught her in an odd moment.