Good advice from andres[i... but won't help the red stain. We have (actually Mrs. C has) a [i]Zuchon] which is an F1 cross between a Bichon and Shih-Tsu... same problem especially since she's a little white dog. Common to Maltese as well.
She has the red staining at her rear but also at the eyes and around the mouth. A product here in the U.S. called Angel Eyes (a food additive) seems to work for treating the eyes, but we've just had to live with the mouth (and other area) stains.
Our vet says all dogs, regardless of color have such yeast induced stains, it's just more pronounced on white dogs.
Additionally, this dog requires grooming about once a month (hair grows quickly, but no shedding) and the groomer takes pains to trim closely around the eyes and mouth, which does help...
We've also found that a paste mixture of equal parts of Milk of Magnesia and corn starch mixed with just enough 3% peroxide to make the paste and applied to the dogs eyes,and mouth (won't work for the rear areas, since they'll lick it off) will bleach it out with no harm to the dog. Apply it overnight and rinse well the next day... may take a few dedicated days of doing the application/rinse procedure to get an adequate cleaning.
Our vet also said treatment with tetracycaline drugs seem sto work a lot of the time, but we didn't think the condition serious enough to resort to antibiotics... Goog luck!