All the ideas above sound great, but this is what I see: I see raw umber and brown for the shadows at the top. Moving downward where the sunlight hits the horse, I see burnt umber and blending downward to a terra cotta color with a lot of white. I don't see sienna or ochre in this, but those would be some beautiful colors to mix in. If you don't have ready made colors, I would put a lot of brown, and bit of white, a smidgen of red and yellow. Mix until you find the right color. Good luck to you. Your paintings are beautiful! Thanks for sharing...color is very difficult for every painter.