The two golden retrievers I know noses have a big brown spot now......I would call and ask the vet, im curious to see what thats' about. Let us know what they say.
the same thing happened to my dog too. she is 5 now and it happened about two years ago. it was black then got a pretty big white spot on it. i never really thought much about it and she has been to the vet and had blood work and things since, and she is perfectly fine. i cant say much about your case (every dog is different) but if you are really concerned you should go to your vet or atleast call them and see if it is worth comming in to be looked at