Well, the 30 Years War you're talking primarily Germany and in the 17th century many well known breeds formed their roots. The German Oldenburg, Hanoverian, Trakhener, Freisian, and Westphalian are the major ones. It's interesting to note that breeding programs of this time were governed by the state, and more specifically the regional governments, so you see organization and documentation but also breeding programs geared specifically for the requirements of the regional community needs and desires. There was also quite a bit of breeding stock importation of Thoroughbreds, Iberians, Selle Francais, and several others.
If you go with any of these, your standard colors of chestnut, bay, black and gray won't steer you wrong.
I looked up a wiki on the war and some participating countries. The Spanish Netherlands might offer some interesting possibilities for horses as well.
But, I digress, I love researching this kind of stuff and can go on and on. I know you just want to paint the horse. lol!
For the basic horsie colors, if you go earthy tones with the rider and a splash of color - if you make the horse a bright shiny red chestnut (looks like brand new copper) that works nicely as a counter to the soft earthy tones. A blood bay also works as well - similar coat color only the mane, tail and legs are black. You've already got the foundation started for both of those colors. And they are easy to do in either pigments, pastels, or acrylics.
Darker horses have a tendency to blend into the muted earthy tons.
Let me know what color you decide.
Cheers,
Karrie