I myself would not use the word "rare"to describe the tri colored yorkie or any other of the "off colored" yorkies (chocolates, goldens etc.) they are not the traditional colors and in the past, they were more unusual, but they are becoming much more common place every day. We are allowed to register them with the AKC however we are not allowed to show them as the YTCA does not recognize any coloration other than the blue/tan.
I would possibly use the word "unique" as a descriptive word in place of rare.. the patterning of each puppy can be very different from each other and some of the variances in color can be be extremely unique and beautiful.
Diana