Foot licking/chewing is the way that dogs indicate that their feet are itchy or painful. The most common reason for chronic foot licking in dogs is allergic dermatitis (food allergy or pollen/dust allergies) with secondary yeast or bacterial infections between the toes. If only one foot is affected, is is possible that an abrasion or trauma initiated an infection. In older dogs, hormonal diseases such as hypothyroidism can predispose to infections and foot licking as well. The brownish discoloration can be due to salivary staining or a yeast infection. I recommend that you take him to your veterinarian for diagnosis and treatment. If all symptoms go away with treatment of infection but then recur later, then an underlying cause must be pursued.
Kimberly Coyner, DVM DACVD
Dermatology Clinic for Animals of Las Vegas