Dog tears and saliva contain an enzyme that causes a chemical reaction on the hair turning it red. There are many products on the market to remove this. They are marketed as Dog tear Stain Removers. The following site has a good chart that discusses these.
The website below has a different approach suggesting altering your dog's pH levels to prevent the staining which you may wish to look at.
There is also an antibiotic that your vet can prescribe called Tylan that has been shown to be useful to stop tears and saliva from discoloring a dog’s coat. You will want to ask your vet about this.
Now when a dog's eyes are tearing and gets a build up under the eye, a yeast infection can result. You will want to clean the area real good, being very careful of the eyes while you are doing this. Trim the hair back and try some miconazole cream which may provide some relief until you can have him seen by your Vet.
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