A dog's nose can turn brown because of a condition called snow nose, which makes the pigment in a dog's nose change as the seasons change, according to WebMD. Snow nose can cause a black nose to become brown in winter and go back to black in spring and summer.
Snow nose, also called winter nose, often affects golden retrievers, Labrador retrievers, Siberian huskies, Bernese mountain dogs and some other breeds.
A similar but unexplained condition called Dudley nose is a type of nasal depigmentation in which a dog's nose can gradually fade from black to brown to pinkish white over time. This type of depigmentation usually occurs on the part of the nose that has no hair. Dogs that commonly experience this syndrome include doberman pinschers, poodles, Irish setters, Samoyeds, white German shepherds, pointers and Afghan hounds.
Depigmentation of the nose is usually a reason for concern only for show dogs, but it can sometimes be a sign of a health problem, so it's best to consult a vet to determine the cause. Some dogs are disqualified from dog shows for having permanent depigmentation, according to the American Kennel Club, but snow nose is not a reason for disqualification, according to Professor's House.