One really quick way to paint horses that I was experimenting with was to paint it a light brown, let that fully dry, then paint it a dark brown and before it fully dries take a tissue and gently wipe away the darker brown, revealing the lighter brown underneath. You'll leave the dark brown in the crevices, giving shading, and the light brown will appear on raised surfaces, but will be tinted with the darker brown.
It's a way I read for speed painting horses by historic gamers, you can probably find a tutorial online. I found using
GW paints, since they dry so fast the trick was to only paint half the horse before wiping away the paint and pay attention to where you started painting first and start wiping that spot first. Otherwise you'll get an inconsistent look.
I then also threw a Devlan Mud wash over it just to emphasise the shading. I'd show you pictures but I don't have any unfortunately. It sounds like a lot of steps, but the thing to remember is they're all "big brush" steps, in other words you can apply the light grey with a big 1/2" thick brush so it's quick, or even spray it, the dark brown again you can apply with a big thick brush and then the wash you can apply with a big thick brush. So even though it's 3 steps, it's 3 quick steps (compared to manually highlighting which takes ages!).
I can't find any pics specifically of the horses, though if you look at my Tyranid pics and zoom in on the background, there's a few Rohan horses I painted using that method.
http://www.dakkadakka.com/gallery/385816-Tyranid%20scheme.html