Horses teeth aren't coated with enamel like dogs, cats, and humans. They are made of a more porous material called cementium (spelling..?) In any case, this more readily absorbs the colors of the foods they eat. Its completely normal for a horse's teeth to be anywhere from yellow to a brownish color. They don't need brushing.
Plaque build up around the gums, on the other hand, can irritate the gums and may eventually cause bleeding. Plaque is thick, hard and yellow-gray in color. It occurs in geldings and stallions as a rule. If you think your horse may have tartar build-up, then have your veterinarian check it out, and he can clean it for you, as it needs little more than a "horsey tootbrush" to get it off.
I would make a tartar check part of your routine veterinarian examination and vaccinations, along with the entire dental check (seeing if the teeth need floated, etc.)This way you save money on vet trips when you schedule all of your horses maintanence needs at the same time.