Do you have a white dog? Are you wondering why his muzzle and paws are turning to a rusty red color? Then I have a suggestion. Nothing scientific. Just my personal observation.
The dog you see in the picture is one of my furry kids. He is an 11 year old, male cockapoo (cocker spaniel, poodle mix). His name is Biko. The picture on the left was taken about 5 years ago. Did you notice his red muzzle? After a while he developed red paws too. The picture on the right was taken very recently. As you see, the red color disappeared. Now I have a white, fluffy puppy. The only thing I changed in Biko's life is his food. Definetly NO CHICKEN! Anything else is OK, but chicken apparently causes allergic reactions. During this 5-year trial, all my friends with white dogs followed my suggestion. Now we all have white dogs with no dirty looking muzzles. If you have the same problem with your dog just try, be patient for couple of months, and take before and after pictures. You may be surprised.
P.S: By the way, I would not mind if my dog's hair color changed to red, purple or green and even if he would lose all his hair and become bald. I love him very dearly. This is deffinetly not a 'cosmetic,' but a health issue. There is no normal chicken left in the world (except the REAL organic ones). We human beings created 'monster chickens' by pumping hormones into their body and using them as a food product. Remember: Dogs do not eat monsters!