Hmmm..... Brushing a horse's teeth isn't something I've seen or heard of people doing. Horse's teeth naturally acquire build up and yellow. Their mouths are not the same as a dog or cat. Horse's teeth are covered in cementum, not enamel like our furry friends. This material is more porous and absorbs the coloring of the food the horse eats. The yellow to brown (sometimes even darker but not often) staining is normal. Tartar can build up at the base of the canines, and sometimes incisors, in geldings and stallions. This material is yellow-gray, thick, and hard. It can cause irritation and even bleeding in the gums if not removed. However, this is done by a veterinarian or equine dentist and should be a part of the routine examination and care of teeth if the horse is prone to tartar (I haven't seen many horses who have a problem with this)...not with a tooth brush
If you feel your horse has some sort of extra build up that it shouldn't have, such as a bacteria or some sort of infection, then you should have your vet take a look. Maintaining a horse's teeth is a normal part of horse care, just like vaccinations, worming, hoof care, and routine check ups. Most horse people try to schedule the examination of the teeth along with the aforementioned procedures such as vaccinations.
When the horse's teeth are growing too long and uneven, the vet will float, or file, the teeth down so that they meet correctly. This is usually the only routine maintenance the teeth need. Other medical procedures associated with teeth include removal of the wolf teeth (a pair of sharp canines, if your horse happens to have them and they are bothering him/her or interfering with riding) or other such things as removal of absessed teeth. Of course, abcesses and other painful teeth problems have nothing to do with plaque build up and you'll notice your horse dropping food while eating, shaking his head, or otherwise acting uncomfortable about the mouth/head area. Hope this all helps!