Black - bold primary colors, red or royal blue depending on the hue of the horse
Bay - hunter green
Gray - lavender or soft blue tends to look nice, or black is always acceptable
Buckskin - black or a dark green
Palomino - not too sure on this one, probably cream
Chestnut - black
With paints and appies, I have a hard time deciding, it really depends on the horse's base color. Nothing patterened though, too busy already!
There is this neato color wheel that has all the horse colors and all the accent colors. You use it to determine which color would best compliment your horse. There is also an interactive application that allows you to try different colors on different horse colors.
So, what's your preference?