Check with your dog's veterinarian. This could be a kind of allergic reaction to something, it could be a sign of an autoimmune problem, or (depending on the breed) it could be a totally normal part of aging.
Be aware that your dog's nose might get sunburned now that it is pink instead of black. Try not to let that happen. I'm not a dog, but I have to imagine that a sunburned dognose could be quite painful and annoying.
If the dog recently began playing with new toys or chewing on new/different things, especially anything plastic, it's possible that it is an allergic reaction.
See this question for more information about the color of a dog's nose:
Posted 428 day ago