There are yorkies that do retain a darker color but not so much the black and tan markings you see in puppies. Genetics do determine adult coat color and texture. Generally a softer coated dog will retain darker color than a silk coated yorkie. Also, most yorkies do lighten in color with age, especially females kept for breeding.
LOL...I was totally the opposite of you. The first yorkie I saw was an adult and I was confused because when I looked at yorkie puppies they were all black and tan, I wanted that adult coat color.
