White is the traditional colour and the one generally preferred yes. You can, however, use any plain, neautrally coloured saddle cloth. Ive competed a few times with black on my fleabitten as it suits him more than white. Just keep it subtle
My preference is for white with a subtle silver hue and Savroski crystals stitched all the way round the outside...
only kidding!!
If you want to fit in with the dressage crowd, then it has to be white, there are no rules about it (apart from plain, neutral colours) so I think you should use whatever suits your horse best!