I think this color wheel is always pretty helpful. They are talking more about what the rider should wear to make the horse look good, but their tips work for what the horse should wear too! :) Hobby Horse Clothing Co. - Shopping
I love the way chestnuts look in navy blue or green, personally.
I think grays look good in pretty much every color. I always decked my gray out in the brightest colors I could find and she rocked alll of them.