At the moment i think it's one texture per horse type per faction. I've created different coloured horses within one faction by editing the following files:

descr_mount.txt has the basic mount list. copy an existing light horse or other and call it
type grey horse
class horse
model horse_grey

Then in descr_model_battle copy and paste an existing horse and call it horse_grey too. make it look to your new grey tga texture when used by the specific faction.

Then in export_descr_units make your unit use grey horse

hope this helps.