What characteristic defines the coat color transition for a Yorkshire Terrier puppy into adulthood?

Puppies born black and tan gradually transform their black areas to steel blue.

One of the most distinctive genetic features of the Yorkshire Terrier involves its progressive color change as it matures from puppyhood into full adulthood. Puppies are invariably born with a coat coloration described as black and tan. Over time, as the dog develops, the black pigmentation on the body undergoes a transformation, gradually fading and altering its hue to the characteristic deep, rich steel blue shade that is preferred in the show standard. This steel blue is genetically distinct from true black. This change is not immediate; the complete establishment of the adult coat color can often be a slow process, sometimes not fully realized until the dog has reached approximately three years of age, though the tan points on the extremities usually maintain their brightness.