Yorkshire Terriers standards state these colors, Black and Tan, Black and Gold, Blue and Tan , and Blue and Gold. Blue is a steel blue, its not to be grey/silver, but that is what it normally turns out to be. Sometimes as a yorkie gets older they turn more white, instead of blue.. YT admin put down tons of information about the standard yorkie.. go here:
http://www.yorkietalk.com/forums/showthread.php?t=16045