Sorry it took me awhile to get back to you. I've been studying like crazy the last few days....
I would wash the base first with dark brown, making sure to get underneath the horses. Fixing them afterwards is kind of a pain. Then I highlight the horses' heads, legs, and rumps. Then I often do another brown wash to make sure that each horse is clearly separable from the horses next to it. Then I paint the main jacket of the rider. After that, I sort of figure out what colors I want to portray as I go along. Especially in the true 2mm (non-Epic) figures you really need to feel it out as you paint, because there isn't much room for detail there. The rider is partially hidden by the horse so you get a lot less space to work with.
The Epic models have significantly more space for detail work. I've actually been tempted to re-do the true-2mm cavalry with more details, but I've hesitated because I don't want to scale creep the true-2mm stuff. Anyway, after seeing your infantry I'm excited to see what you manage to come up with for the cav!
Designer and owner of Forward March Studios: The World's first comprehensive range of 3D printed historic miniatures.
TBH I have a mental block starting the mounted, but typing the question out helped me figure out a way. I've moved on to prepping artillery and train so the horses will be later. Also I has to go back to work today after 8 weeks off, so my painting time is less now.