A description by the AKC of what defines Yorkies. Here it is:
The Yorkshire Terrier is a tan dog with a blue saddle. Particolors exist, although they are not correct for the breed standard. The particolor coat is white with black/blue and tan. It is very rare to get a particolor, and if one is found, it tends to be very expensive.
[12] Some Yorkshire Terriers are liver or chocolate, a brown color; they are unable to produce black pigment.
[13] The breed is defined by its color, and such non-standard colors may indicate health problems or cross-breeding with other breeds of other colors.
[4] The AKC registration form for Yorkshire Terriers allows for four choices: blue and tan, blue and gold, black and tan, black and gold. Color alone will not affect whether or not a dog is a good companion and pet. Even though off-colored Yorkshire Terriers are advertised at premium prices, being of an unusual or untypical color is neither new, desirable, nor exotic