According to Mercola, your dog's nose may be changing from black to pink due to changes in the weather, or it may be caused by the aging process. Certain dogs are prone to "snow nose" or "winter nose," where their noses lose pigmentation during colder months.
As warm weather returns, so does the darker coloring, indicating that the enzyme tyrosinase, which develops pigmentation, works better at higher temperatures. Mercola notes that this enzyme is thought to be responsible for the fading of color in the noses older dogs; tyrosinase seems to lose its function as dogs age.
California's Rainbow Vet Hospital identifies several other reasons for a lightening in the color of a dog's nose and recommends consulting a veterinarian to rule out serious causes. Sometimes an inflammation or an infection makes a dog's nose change color. Allergies are occasionally the cause of a color change, particularly if the dog is sensitive to the plastic in its food bowl or the food itself.
Some dogs who spend a large amount of time in the sun have noses that fade, according to the Rainbow Vet Hospital. In addition, certain serious autoimmune diseases cause dogs' immune systems to strike out at their own cells and wipe out pigmentation.