Staining found beneath the eyes and around the mouth is unhealthy and can be dangerous.  The reddish-brown stains on the face result from the hair constantly being damp or wet. This is the ideal environment for bacteria and a common yeast infection called Ptyrosporin (Red Yeast).

Advanced pet grooming products contain enzymes that work on these stains and optical brighteners that improve appearance.  The primary goal is to keep your pet's face clean and dry to avoid infections and the resulting stains.  Whitening products formulated by #1 All Systems eliminate face stains and improve coats by removing stains, yellowing and discolorations.
 
From Amazon.com
Reviewed in the United States on January 28, 2020
Verified Purchase Customer
5.0 out of 5 stars 
Best Product Period
Adopted a Bedlington Terrier had mouth and tear stains
Tried a bunch of highly rated snake oils offered in Amazon...Ivan say don’t waste time or money.  This stuff delivers.
I watched professional groomers using this and after getting my order I see why.  There are visible results after a couple of uses.
No stains with regular use.
I know that the others I purchased had reviews similar to this and that’s why I bought them but research grooming videos and you’ll see this brand used by people who show dogs