Why is my white puppy's pink belly turning white?

I have a 5 month old lab mix puppy. Normally he has a very bright pink belly. It is starting to turn white-ish. I am not sure if it is normal since he is a cream/tan/white puppy? His tongue, ears, and between his paws are still very pink. He has a normal diet of Purina One Large Puppy Breed twice a day. Is this normal? Thanks