I found this while searching for a reason her beard is stained. I believe it is due to eating the beef dry dog food. So now she is on Blue Buffalo Chicken.

Beard stains, "Beef stains" and/or "Red Beard"

"My Schnauzer's beautiful white beard is turning brown! What can I do?" is a common lament we breeders hear. This reddish or brownish staining of the beard, beard stains and sometimes of the hair on the paws or leg furnishings as well, is usually seen in salt & pepper and black & silver Schnauzers, but may even be seen in solid black Schnauzers. Occasionally referred to as "red beard" or "beef stain," this hair discoloration can be annoying and difficult to correct. Observing your Schnauzer can provide clues to behaviors, activities, or external causes that may be producing the stains. Some of these causes are discussed below, as well as possible remedies. Just as there is no single easy method for removing stains from your clothing, there is no simple answer for the removal of stains from your Schnauzer's beard and leg hair. There are, however, several home remedies and commercial products available that may help. Please remember that these are only suggestions, and they may take repeated applications. Also, since staining may originate from a variety of sources (food, water, saliva, soil, etc.), these remedies may not solve your individual problem. They hopefully will improve it, however, and with luck may possibly even eliminate the stain altogether.

First, try switching your dog's diet to one without beef and containing a different protein source, such as chicken, lamb, or turkey. This may help by eliminating the cause of the stain. Red beet pulp as a diet additive has also been implicated in staining. Try changing to a food without artificial colors, flavors, preservatives or other additives.

The pH of the dog's saliva is also believed to be important. Since water as well as diet may play a role in such staining, a change to bottled water may help. Additionally, apple cider vinegar (NOT white vinegar!) added to the dog's drinking water (1 teaspoon per quart of water) may be useful. Note: If your dog is prone to bladder stones, please check with your veterinarian before trying this.

Dirty teeth can be a source of beard staining, as well as of bad breath. Check your dog's teeth often for tartar buildup. Dirty teeth can also contribute to numerous health problems. It is important to keep your Schnauzer's teeth clean, not only to help keep his beard white and his breath fresh, but also to promote good health. Frequent brushing (with doggie toothpaste only!) will help keep his teeth clean, but occasional professional dental cleanings by your veterinarian will sometimes be necessary.

To remove existing beard stains, some of the following suggestions may prove useful:

Wash the beard once or twice a week in a mixture of white vinegar and water.

In a bowl, mix together equal parts Hydrogen Peroxide (from the grocery or pharmacy; make sure it is fairly fresh), milk of magnesia, and corn starch (to give the mixture the desired consistency). Combine only what you are going to use immediately -- it needs to be made fresh each time. Brush this liquid paste on, using an old toothbrush. Put the dog in a crate so he does not get everything messy and try to make sure he does not lick it off. The paste will dry to a powdery consistency after about an hour or two and then can be brushed out. The stain, especially if it is a dark one, will not necessarily be gone after one application, but definite improvement should result. This treatment may need to be repeated a few times. Be sure not to get the mixture on areas of the coat you do not want bleached!
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Mix together equal parts lemon juice and table salt. Then mix in one beaten egg white and apply to the beard. Leave the mixture on the hair as long as desired. When dry, it can be brushed out.

Available at pet stores and through pet mail order catalogs is a commercial shampoo called "Snowy Coat," by Lambert Kay. It contains bluing (not bleach), an optical brighter. Work a small amount of the shampoo into the beard and let it sit for a few minutes before rinsing out. Similar shampoos may be successful in individual cases: "Shimmering Lights," "White On White," etc.

Try adding a drop or two of "Mrs. Wright's Laundry Bluing" to the conditioner used after you shampoo the beard -- it may help. You can add the bluing to the bottle of conditioner, and use it on the beard and legs each time you shampoo.

"Fearing Super White" is a product available in most horse feed/tack supply houses and pet and veterinary supply catalogs. It is purple and has a raspberry scent. Work a small amount into the beard to form a foam and allow to set for about 3-5 minutes, then rinse out completely.

For prevention of discoloration, use of a light coat oil (such as "Purepet Coat Oil") will fill hair pores and protect them from filling with unwanted pigment.

Finally, food or environmental allergies may be playing a part. Corn and wheat in the dog's food may contribute to allergies, which can lead to discoloration of the feet when the dog repeatedly licks them due to itching. If switching foods does not help, and you suspect your dog has allergies, consider having your veterinarian perform some allergy testing.

Under no circumstances use a human bleaching product (hair lightener or highlighting kit) on a dog. A dog's hair pH is different from that of humans, and the fur may not turn the desired color -- not to mention that such a product could be harmful if the dog ingests it.