Brown looks lovely on a bay, and if you're buying new, and plan to do quite a bit of showing, brown is the way to go - it can be used in all disciplines. The only time I'd consider black is on a piebald or true black horse.
I nearly converted to brown, but I honestly prefer black. My pony is a bright red bay with black points and black does seem to suit him. I think that most judges would look at the horse first, and the tack and riders clothes second. I would only think it would matter if there was very little to choose between two animals.
Vaguely funny story - I was at a show and an saying that I had finally got the dosh for the saddle of my dreams an Albion selecta (second hand). An acquitance then started to bang on about having spent £1,200 (what my pony cost) on a brown saddle as she needed it as she has a reg. cob (unlike mine) and they show county dont you know. She came seventh and we came third which sort of summed it all up really!
I use black tack on my bay - I think it goes really well with his black points. A judge who would mark down for black tack would not be much of a judge anyway. There are far more important things to look at than the colour of the tack!
well with things i read in some posts it seams things differ to where u may live, i spend a lot of time at shows even help out and what im told and what i see is different to what i read. i dont do sarcasm