You can't really put too much stock into the coloring of a Biewer pup as a definate indicator as the coloring changes as they get older. For all you know, you could be dealing with a shady breeder who is trying to sell you a Yorkie / Shih Tzu cross but claiming it's a Biewer or something.
Your best bet would be to ensure you are dealing with a reputable breeder, find out info regarding the dogs' pedigrees and research them first.