This should make things clear:
Colors
Puppies are born black and tan and are normally darker in body color, showing an intermingling of black hair in the tan until they are matured. Color of hair on body and richness of tan on head and legs are of prime importance in adult dogs, to which the following color requirements apply:
Blue: Is a dark steel-blue, not a silver-blue and not mingled with fawn, bronzy or black hairs.
Tan: All tan hair is darker at the roots than in the middle, shading to still lighter tan at the tips. There should be no sooty or black hair intermingled with any of the tan.
Color on Body
The blue extends over the body from back of neck to root of tail. Hair on tail is a darker blue, especially at end of tail.
Headfall
A rich golden tan, deeper in color at sides of head, at ear roots and on the muzzle, with ears a deep rich tan. Tan color should not extend down on back of neck.
Chest and Legs
A bright, rich tan, not extending above the elbow on the forelegs nor above the stifle on the hind legs.
http://www.akc.org/breeds/yorkshire_terrier/index.cfm