Color changes like you described are totally normal and expected. Yorkies are a breed that has a color dilution gene, it's very fun to watch and see how much they change. The first time I went to the groomer with my first Yorkie at 9 months old I thought they were bringing me the wrong dog when they brought her out because they had clipped off all her black hair and she was silver-blue underneath!! It was a shock when I didn't know to expect it.
Teresa