Dogs sometimes lose the pigment in their nose. This can happen during their puppyhood, but it happens more frequently as the dog ages. Loss of pigmentation can be due to their age, the weather and temperature, a skin condition, or even a reaction to something as simple as the dish they eat out of. Many reasons for loss of nose pigment are perfectly natural and no reason for pet owners to worry.
Switch your dog's dish. There is a condition called Plastic Dish Nasal Dermatitis that occurs if your dog eats or drinks from a plastic or rubber bowl. There is an antioxidant in the plastic that triggers a reaction that causes a loss of pigmentation.
Keep your dog in a warmer climate. Many dogs, including Labs, are subject to to Snow Nose or Winter Nose. When the weather is cold, some dogs' noses change from black to pinkish or brown. As they age, the enzyme tyrosinase that causes pigment, doesn't work as well in lower temperatures. When the temperature rises again, the enzyme works and pigment returns. Keeping dogs inside will help keep their darker pigment.
Cosmetically alter the pigment of the nose. Trainers and people who own show dogs sometimes have their dogs' noses altered by way of black tattoo ink.
This often happens with collies that get dermatitis on the nose that causes blistering. The collies are kept out the sun, treated with a steroid, then their noses are blackened with a special tattoo ink.
If your dog has Vitaligo, which is a natural loss of pigment on the nose and/or lips, there is no treatment. You can have your dog's nose cosmetically altered, but because Vitaligo is not a disease, you cannot medically treat it like one.