Here's another link to a photo of a dog with sooty hair on the head (black hairs mixed in with the tan). From what I can tell from the literature, this is considered a "fault" according to breed standards, but they often grow out of it. There's also a Coat Color thread on Yorkietalk I'll link to below, too.
JaLa Yorkshire Terriers - JoJo http://www.yorkietalk.com/forums/gen...rt-thread.html