One of the few things I consider myself an expert on is color coordination.
From office supplies to clothes and everything in between, I color coordinate it all. When it comes to horses, certain colors look better on certain horses. If you already have it figured out, you're lucky. If you're stuck, here's a basic guide to match with your horse like a pro.
What to have in color: saddle pads, boots, wraps, polos, bonnets, etc. If it's cloth, it can be colored. In terms of the rider's outfit, definitely color the shirt. Gloves are a possibility, too. Avoid having colored pants, unless you can pull them off (I can't).
What not to have in color: Your saddle and bridle (with the exception of some western tack). Your helmet, if you're over the age of 18. Your boots.
Bay/Brown: LITERALLY any color will look good on your horse. I prefer white. There's something about white boots on a bay horse that make them look extra athletic. The downside of course is that your clothes and tack might change colors. They'll probably turn black, and your white saddle pad will probably look like a fur blanket. I've also seen some bays rocking purple and looking fabulous doing so. For lighter bays, black is a good look.
Chestnut horses: Since there are so many different types of chestnuts, picking a perfect color can be tricky. Generally, it's hard to go wrong with green. A chestnut in hunter green gear is classic. For a liver chestnut, white looks great, or a bright blue.
Black horses: Here's where things get crazy. With black horses, you can play it safe with royal blue.
Grey/White: RED. EVERY. TIME.
Palomino: The easy choice is black, but turquoise looks amazing if you can find it.
Colored Horses: Paints, Appaloosas, Roans: Dress your horse in black to put the spotlight on their beautiful coat. When your horse is already colorful, you have to do little work.
Putting it all together: after you've picked your horse's color, you need to make sure you can match him. Find an athletic-type shirt in their color and pair it with a pair of breeches. I usually wear either tan or grey breeches to keep it simple.