Usually by the time the pups are 12 weeks old I can tell if they will be the blue and gold. If I can't tell for sure of the color by the time they are sold I just suggest the puppy buyer wait until the pup is older before marking the color. I have bought yorkie pups in the past that were listed as black and tan on their papers but they were actually blue and gold.
Here is a interesting link that explains the different breaking patterns of puppy coat and colors:
http://www.yorkietalk.com/forums/gen...rt-thread.html
Are your pups starting to break on their head yet? I have some 8 1/2 week old pups and their heads are breaking to dark gray.