I have yet to see a horse that didn't look good in a royal blue color. It works for mares or geldings of any age and any color. Not to mention that it's easy to find matching tack when you choose a popular color like that. Some colors like teal are hard to find and even harder to match because different companies have different shades.
well I have a chestnut and I rode him in blue and then changed to green and pink. If your horse is a lighter chesnut I wouldnt recomend pink at all but blue or green and red might suit. Even puple. Personally I like pink and green
I looked after and rode a chestnut for a friend for a while, and we found that sarah looked good in the olive green (well I think that's what you would call it) and a light/babyish blue... But because I'm hopeless at describing colours I've uploaded some pics to show them.