My (old) favorite horse color: Rife Butt (Howard Hues) + Tanned Flesh (GW). This combination produces a Chestnut color which varies from a dark brown up to a warm brown/red. Adding Graveyard Bone (Reaper), creates a chalky brown/grey which could be roan or grey.
I think both my Rife Butt and Tanned Flesh have both dried to mud at this point. Ruddy Brown (Reaper) is the base for my most recent unit of cavalry. Also from Reaper, Heartwood Brown for dun, Fair Skin for tails/mains, and Dusky Skin for grey. Foundry triad Chestnut and TerraCotta have joined my painting table. Expect to see them used in the future.
A pair of handy reference pages.