http://www.snafflez.com/horsemaker/horseshowcolours.html
This website is amazing when it comes to looking at how well colors go with a certain color horse. :]
I ride a bright chestnut OTTB gelding, and we have a /lot/ of chestnuts at my barn. We have a tendancy to play around with them and dress them up in different colors.
My personal preferance for chestnuts is always bright colors; bright purple, bright blue, lime green, and even some pastel or lighter colors; a light sky blue, lilac, rosey pink, mint green, etc.
A lot of it depends on how saturated your chestnut is; there's a liver chestnut at my barn who looks adorable in light pink pads and boots, and the bright chestnut I ride looks best in bright blue or green. :]