I haven't painted any horses yet but I have several books on painting animals so I am going by what I think the books would say lol
Base in the lightest reddish brown color, then shade in the darker reddish brown areas. You could do more than 2 shades if you need to, but start with the lightest. Then stipple on a light grey, make sure the stippling is different size dots.
You have a beautiful horse! Thats one of my fave horse colors.