So anyway....
I haven't read this entire thread, because it seems to have gone off track. I'm always interested in hearing about white yorkies, because I am pretty sure that in a few years, Thor will be completely white. He started off as a black pup just like any other yorkie, even at three years old, his coat continues to lighten. When I got him at two, he had some steel blue on his shoulders, but that is now a very light blue, and his butt is basically white.
I've seen on other yorkie in the flesh with the same coloring - he was completely white, with a few black streaks. I haven't seen this gene expression described anywhere on YT, so I am curious to learn more.
The hair itself is beautiful, super-soft and shiny. A passerby recently described him as "luminous"!
The OP's pups seem a little different to me, because the pup was born white, while Thor was not.