My cross country(and showjump I guess) colors are royal blue and black(: Im still a junior and I only compete in low level events. I find them a good medium between fun and classy.
For dressage I always wear white pants, white show shirt, black coat, black belt, black helmet and black tall boots. The pony has a white saddle pad(and we are lucky enough to have seperate dressage tack, but that's not nessacary at low levels)
im my pony club we are REQUIRED to show in white/black with the exception of the navy polo shirt.