This is based on the generating cycle of elements. It is important to choose the right horsey. Samurai also flew flags from their horseys based on this method of calculation.
Certain members might enjoy this video.
Virgo
Horse Riders and Choosing the Correct Color of Horse
Re: Horse Riders and Choosing the Correct Color of Horse
Of course, this is just good general information, it doesn't have to be based on "warfare" and so on. You can just choose a horse that generates your life elements, and so on.
Virgo
Virgo