When and even if a yorkie coat changes depends on what type of coat the yorkie has. A soft dark coat may never change where as a true silk coat will start to "break" as a puppy, at least that is my understanding. Generally an adult yorkie coat will not be in until 18 - 24 months and may continue to change slightly (usually going lighter) as the dog ages.
Others may have a better answer for you and will post soon hopefully.....as I'm not an expert by any means!
