I've been reading this thread with interest. I'm not sure if this is helpful for you, but since you were asking about gray/roan, I just bought a TB mare registered as gray or roan. Here is a picture of her coat.
Summertime, before I bought her:
She looks more less bay and more gray to me now. Here's a picture from over the weekend, I don't have one without her coat on that shows the red hairs:
I don't know what any of those genetic terms mean at all.. but I did find the link from the other page on rare colors of TB's very interesting!