Well I know at the rated shows I go to in the US people always use a white pad. The only exception I've seen to this is if a person has a grey (almost white horse in appearance) then I've seen them use a black pad or a white pad with colored piping.
My experience with this kind of thing has been interesting, since I ride my horse in a special thinline black fitted dressage pad. Which I was convinced to buy at a saddle fitting clinic for my horse by the clinician and my trainer. I wanted to get the same saddle pad in white but the clinician had only brought black saddle pads with and my horse needed it right away since she was showing all signs of having a sore back plus we had a show coming up in a month. The clinician said she was completely out of the white and it would atleast 4 weeks, well my horse needed that pad that day regardless of the show. The clinician was from europe and she convinced me the riding with a black saddle pad at a dressage show was on the rage. My trainer also chimed in at that point and said it would be fine. So we bought the pad ($150, not even a full square pad like I wanted). I new I could have ordered the pad from the web but then I would not have had it that day or the inserts/shims and would of had to pay shipping so I did what was best for my horse. When we got to the show the next month I went to saddle of with my black saddle pad and my trainer goes you can't do that, where's your white saddle pad? I ended up riding with the white saddle pad on bottom and then my black pad so that the saddle was kept in the right place. I'm sure the judge must of that was weird. No one else at that show had a black saddle pad. Sorry for the novel. This is my saddle pad: Cotton Comfort Fitted Dressage Pad - Horse Rider Performance Saddle Pads - ThinLine