I've noticed over the last few months that my long-haired chihuahua mix's nose has been gradually lightening from black to brown with sort of a mottled effect. I've been researching this online and the general consensus seems to be that it's normal, but usually it's in reference to a puppy or young dog reaching adulthood. Mine is a rescue so I'm not sure how old she is, but probably something like 5 years. She also, before she came to live with me, used to be chained up outside 24/7, whereas now she just goes for walks twice a day (very short ones lately, as it's been quite cold), although she does spend a lot of time on a glassed-in balcony.
She's not exhibiting any other problems -- she's energetic, affectionate, seems happy, has a long, lush coat and bright eyes -- except that she doesn't seem to be eating much. (That's a whole other saga.) And the other day I found a small bump near her right foreleg about the size of a pimple or mosquito bite, although it's too cold for mosquitos right now. Oh, and she was just recently spayed, if that can have any effect. Her food and water bowls are ceramic.
I live in Korea and although I have a vet whom I like and who seems to like animals (you'd be surprised how many don't here!), but the language barrier is quite a problem and he always wants to give her shots, even when she's just having her nails clipped. So I'm hoping it's nothing to be concerned about.