I've seen this question asked many times. Did you ever see a woman who was growing out her grey hair? It's the same thing. The roots are light and the ends are dark so she'll look like she balding. Don't worry, your yorkie is perfectly normal. I don't know if they actually shed their puppy coat or if they just grow their hair longer.
Yorkies change a lot as they become adults...
Here's a couple pictures of Debbie: |