For 99.99% of Yorkies that have the typical changeover, it can bring about varying levels of intensity. It can range from light to dark...And dark will often be mistaken for black, as the tint will only be noticeable in bright sunlight.
The tan of the will slowly change to a tan/gold. Gold is simply a richer shade of the tan. The tan hair on the dog will be darker at the roots and lighten as it reaches the tips.
When a Yorkie is transitioning from puppy to adult, all four colors may be present as the black is being replaced with blue and the tan is being replaced with gold. The individual hairs do not change color; new hairs grow out.
When people say that this breed does not shed, this is not entirely accurate since of course puppy hairs fall out as the adult coat grows in. If this natural process did not happen, a 4 year old Yorkshire Terrier would have the same coat of hairs as he did as a puppy... can you imagine how unhealthy that coat would be? The coat - much like your hair - is in a constant state of renewal: Growth, rest and fall. Since this happens slowly over a longer period of time in comparison to his heavy-shedding counterparts that have fur, generally you will not notice clumps of hair or anything to that effect.