From a 1970's or 80's UK mag Military Modelling, a tip I always used to use with some success.
1. Undercoat with a white primer, either use an acrylic or seal with and acrylic varnish
2. Take a dark chestnut brown oil paint, I used to use burnt-umber quite a lot, and apply liberally straight from the tube, no mediums, drying additives or thinners.
3. Dab off the excess with a fine grade synthetic sponge, don't wipe, dab.
4. Bend darker oils around the ears, nostrils, muzzle, and the fetlocks.
5. Allow to dry.
6. Fine details like eyes and hooves after the oils are dry, which will take a while.
Give it a go, if you don't like it the oils will clean off the sealed acrylic undercoat quite easily.
Makes horse painting a simple task for those of us that can't paint horses
HTH.
Regards.
Les/J99 ex-Jackal Squadron Webmaster