I think your Yorkie may be a chocolate Yorkie. We have many threads about Blue Born. I do not think your Yorkie is that. When a Yorkie is Blue Born he is born gray with light tan markings, their eyes are either green or blue. This is a color mutation. As they mature they often turn white.
Normal Yorkies are often referred to as Blue and Gold, Blue and Tan. For a mature Yorkie the preferred color is when the black fur turns into a deep steel blue, grey color and the tan is golden cream tan. Check out our YT library for more information. Also check out Snow Blue Yorkies on the web for more color information.
http://snowblueyorkies.com/