Found
this for you, I used this when I started off painting cavalry... the original site doesn't seem to be working so you might have to log on with a google account!
Edit: If that doesn't work, I've copy-pasted the most important part:
Basecoat the horse with an "appropriate colour", possibly give it a
wash of ink and highlight it with "yet more appropriate colours".
Quite vague instructions, huh? Well, the colours of the horses vary a
lot while the riders have more or less similar outfits. Some colour
schemes you might want to use are (note that all inks should be
thinned):
Bestial Brown - Vermin Brown - Snakebite Leather (the GW guide).
Scorched Brown - Black Ink - Scorched Brown for very dark brown
horses. Substitute Codex Grey for dark grey horses. You may also
leave it as it is after the ink wash as in the picture above.
Chaos Black - Scorched Brown - Bestial Brown for dark brown horses.
Chaos Black - Scorched Brown - Dark Flesh for dark reddish brown
horses. See picture below.
Scorched Brown - Dark Flesh - Vermin Brown for brown horses.
Scorched Brown - Bestial Brown - Snakebite Leather also makes
brown horses.
Bubonic Brown - Bleached Bone - Bleached Bone/Skull White mix for
light brown horses.