I have a chestnut and all of her stuff is Lime green or Neon green!! It really stands out. My best friend also has a chestnut and her stuff is Light blue and that stands out too just not as much.
Here are pics...
Sorry but this is the only one I have of my friends horse..ha ha ha
For chestnuts Im thinking lime green, light green, light pink, yellow or orange would look awsome.
Or whatever suits your personalitly.
My pony looks really good in purples but I hate plain purple so I went with a lot of lavender stuff so I wouldn't look the same as other people.