It is not uncommon to see English Retrievers with a brownish colored nose. This happens more often during the winter and is commonly referred to as “snow-nose.”
Snow nose is more prevalent in light and white coated breeds and is not completely understood. Even though it is called “snow-nose”, it can happen in warmer clients as well.
A brown nose can also be caused by using plastic food or water bowls, and is not a health concern. This can be easily remedied by switching from plastic bowls to glass or stainless steal.
Some believe that the real culprit is a breakdown in an enzyme called tyrosinase, which is what makes melanin. Melanin is what gives pigment to the hair, skin and parts of the eyes.
This enzyme is temperature sensitive and gets weaker with age. This is why you may notice it more so in older dogs.
There are obviously other reason’s(allergies, vitiligo, bacterial infection, etc) that a dog’s nose can change color, but normally these conditions present with additional symptoms.