In any purebred breed, a colour(s) may occur that are considered wrong and undesirable. The Yorkie breed standard does not include chocolate, blonde or particolour. Any that occur are considered undesirable and incorrect for the breed. The Yorkie was never intended to be produced in 'many colours'.
So, when and if a reputable breeder gets something in their litter that is so wrong colour such as chocolate, they are given to friends, family or sold for a much reduced price for pet only,spay/neuter.
I would avoid breeders charging inflated prices for wrong colour calling them rare.
You can go to the Yorkshire Terrier Club of America website at
www.ytca.org there is a tab there about wrong colour likely under frequently asked questions.