The American Kennel Club has clear standards on the coloring of Yorkshire terriers. Tan, blue and black hair in a specific pattern are outlined in the AKC Yorkie standard. Mother Nature is an artist who enjoys mixing up the colors every once in awhile, but there's no denying that Biewer and parti Yorkies are Yorkies, even with a bit of white in their coats.
"Biewer" and "parti" Yorkies are have spotted coloring that includes large amounts of white rather than the specific non-white colors that the AKC asserts must appear in a certain pattern on the body for a Yorkie. While the AKC will allow a small patch of white on the chest of a registered Yorkie, it has to be less than an inch in size, so a large amount of white on a Yorkie's coat is one factor that makes it either a Biewer or a parti. The patchwork appearance of the coloring is also a consideration that separates the two from a standard Yorkshire terrier.
The origins of Biewer and parti Yorkies are the most notable difference between the two. Both are off-shoots from the original Yorkshire terrier breed that originated in England. Biewer Yorkies, however, are a fairly new breed that comes from a specific German line. While any Yorkie can carry the recessive gene that causes spotted patterns on the coat, only those that come from the original German puppy's line can be considered a Biewer Yorkie.
Biewer and parti Yorkies are recognized as purebred dogs, but Biewers aren't recognized by all registries in the U.S. They do have an official club and are shown in Germany, the country of their origin, and they participate in shows in some countries, but not in the United States. Biewers are becoming more and more popular in America, though. They are recognized by the American Canine Registry, which relies on the Biewer Yorkie Association for standards of the breed. Parti Yorkies are not considered a breed apart from Yorkshire terriers; "parti" describes the coloring and not the breed. Parti Yorkies can be registered with the AKC and other purebred canine registries, but partis cannot compete in AKC shows, as their white patched coats automatically disqualify them.
If you are drawn to the huge personality that comes in a tiny Yorkie package, the color that he sports shouldn't deter you from adopting one into your home. All dogs have their own personalities, but both Biewers and parti Yorkies have a classic Yorkshire terrier personality. They are affectionate, playful, loving and essentially even-tempered little guys who enjoy being with their humans. The appearance of white in his coat is just one more feature that makes your little guy an individual.