Grays look good in everything! Pick your very favorite color and it will probably work on her. If she's got a lot of dappling, I would stick with solid colors rather than patterns, but that's just me.
When I was 12-13 I started leasing a dapple gray mare. Someone asked me what her color was going to be, and I was dumbfounded--I was new to riding and had never thought of picking a color scheme for a horse's tack before. I thought to myself, "everything looks good on a gray... so I choose RAINBOW." HA! Monti is now a lesson horse for little girls, and still has her silly rainbow halter and lead rope, her silly rainbow nylon reins, and her silly rainbow sport boots... but I kept the silly rainbow swirly saddle pad. I put it on my BO's horse when he asks me to tack her up :P