No, I found the full study. Would take me at least a week to try to understand all of it, but from what little I could get, there is more than one gene pair influencing the expression of white in yaks. Think of eye color in people. Brown is dominant over blue, You need 2 recessive genes to have blue eyes. Simple. So what is hazel/green? At least 8 genes involved in hazel/green eyes! I think something similar happens with the Royal in yaks.
Remember I put the disclaimer in my explanations, that we were depending on the info from that one link. I thought it was too simplistic, so was a little suspicious. I'm not surprised there is more to it. Unless someone puts up the money to sequence yak genes, you are probably going to have to wait for some student to do it for a thesis.