"Conservative Colors" are exactly what everyone above said, however in rated horse shows there's certain colors to avoid in certain divisions. In hunter classes, the horse is being judged, so your coat can be any sort of classic color: navy, black, brown, hunter, grey... lol I guess that's all of them! But you can pick a jacket with a subtle plaid or a herringbone fabric. I also like to coordinate my jackets with my horse's color, and chestnuts look amazing with a hunter green coat.
(Come equitation classes, where the rider is being judged, you don't really want to stray from the navy blue, hunter or black area. This also goes for the open/pleasure classes. In most jumper classes you technically don't even have to wear a jacket, but I ask my students to do it anyway out of respect for the judge.)