I've never had to deal with a grey horse, but I do have a brown with lots-o-chrome. In all seriousness and honesty, his white legs are never brighter than after he comes in from the pasture caked with mud. I let the mud dry completely, and then clean the dry mud off - I swear, any yellow/green/brown stains underneath come right off with the mud, and I'm just left with a little dust that a damp cloth will take right off.
I've also made a thick, muddy paste with cornstarch and water to glob onto stained white markings. Just glob it on, making sure it's thick enough to not run absolutely everywhere, and let it dry. Clean off the dried paste with a stiff brush, use a fine brush or damp rag to get rid of any remaining dust, and that white hair will sparkle!
For grumpy ponies in the show ring, I like to have a crinkly something in my pocket to catch their attention. Nothing really loud that will attract undue attention to you, but just a little something that will catch your horse's interest. A few years back, the standard method in my area was to grab a little handful of arena sand and slowly sprinkle it in front of the horse's nose to catch his attention. That works, but it's very hard to do discreetly, and has really fallen out of fashion recently. I don't think I've seen any "Sand Sprinklers" at all the last couple of years. It's almost all tongue-clicking or a bit of stiff cellophane in the pocket.
Good luck, and enjoy your show!!