There is a huge difference between Biewer and Parti. Biewer were specifically bred with their "standard" as having a definite color in certain spots, and they originated in Germany. They are trying to be registered as a separate breed in this country.
The Parti, on the other hand, is a Yorkshire terrier who just does not meet standard coloring for the Yorkshire terrier. It has something to do with a recessive gene. It is a really easy color to achieve, and the standard coloration of yorkies, with the metallic looking hair, is much more difficult to achieve. That's why you won't see a good breeder selling Parti Colored yorkies. For more information see:
http://ytca.org/faq.html#C This is not to say they won't be suitable pets, but in my opinion they should not be bred.