Yes, it is perfectly normal. There is nothing to be concerned about, its the natural process of the Yorkie growing into adolescence and loosing their puppy fluff and getting the more silky thinker hair. Yorkie's will go through two to three color changes in a life time the older they get.
Here is a thread and a web site that may help you understand better:
http://www.yorkietalk.com/forums/pup...rt-thread.html When Do Yorkies Change Colors? | eHow.com