Breed coloring (byb or genetics)?

I think that, although your dog may be registered as having come from two conformation titled Yorkshire terriers, a dog other than the one on the papers was the actual sire of your dog. The Yorkshire terrier doesn't carry a broken color gene; it only comes in blue & tan. Some other male dog got under the fence or in the door or something, mated with the female and produced a puppy. It is technically possible for every puppy in a litter to have a different father. That would easily explain the genetics of an off-colored pup in a litter of a breed that only comes in blue & tan.