Why is my dog's face turning brown? What's causing the discoloration?

The brownish-red discoloration on your dog's face looks most like salivary staining. The enzymes in the saliva interact with the fur, causing this type of reddish-brown staining. Salivary staining generally occurs when a dog licks a particular area excessively, like the paws or groin, or when they drool a good bit. It's especially noticeable in white dogs, like your adorable pup. Some dogs may drool excessively if they're nauseous, so keep a close eye on your dog for changes in their appetite, vomiting, or lethargy, as these could be signs of a medical issue. If your dog is otherwise doing well, allergies are often to blame (particularly this time of year). You can try bathing your dog with a gentle oatmeal shampoo. It's best to seek veterinary attention if you see this discoloration elsewhere on the body, as this indicates that your dog is itchy if they're licking a lot, and would likely benefit from additional treatment. I hope that everything goes well!