Here's a link from Wikipedia, they state:
"Breeders of Yorkshire Terriers generally agree that breeding of such off-color dogs should not be repeated. Breeding of non-standard Yorkshire Terriers is usually done only by backyard breeders and puppy mills.
The "Chocolate Yorkshire Terrier" is not considered to be its own breed distinct from the Yorkshire Terrier. It is not recognized by any kennel club and it has no breed club; the same is true for brown and red Yorkshire Terriers. The off-color is a mutation to the Yorkshire Terrier breed and not in line with the breed standard."
Chocolate Yorkshire Terrier - Wikipedia, the free encyclopedia.
The only links that I can find that encourage this type of breeding are from people who are selling the puppies, not a good source.