I've worked a little bit more on this - put in a bit of background that I think might work and I'm still working on Billy Bob's face. His mane was really out of control when I took the reference picture and I'm going to try to keep it like that but it does look a little strange in front.
Billy Bob is an Appaloosa. When he was young he was a coppery colored bay with some white splashed on either side of his hips and that was it for white. For a while it was really noticeable how white he was getting when he would shed his winter coat. Now it's not so noticeable because he is more white than that copper color but I'm sure this spring he will have even more white hair. I think he is trying to keep up with me ;-)
I won't be able to work on this any more until the weekend - I have to work all day tomorrow and staff a council meeting tomorrow evening. It will be a long day in many ways. It's hard to start a painting like this and then put it down to do things like sleep, work a regular job, etc. I envy you full-time artists out there!