The lab part of the breed is prone to nasal depigmentation which is when the black nose turns a lighter color, like brown. It does not always mean there is a health problem. My Goldens nose changed about the age of 2 1/2 years and now at age 9 it is going to speckles of black. I am going to direct you to a site that explains different causes for this. IF you notice any illness along with the color change then I would have the dog examined.