Horse coloration is its own science and art. On the science side the genetics of most horse colors has been documented, and breeders can plan to get the colors and patterns they want, with fairly reliable outcomes. On the art side, there are strong cultural preferences for different colors.
Some breeds of horse are required by tradition and registry rules to be certain colors: the Friesian must be black, the Exmoor may not have any white markings.
Other breeds - the Icelandic for example - allow a wide variety of colors, from solids to spots.
Here are some interesting sites about unusual horse colors:
Morgans - Morgans are commonly bay and black. But they come in buckskin, grey, palomino, sabino...there have even been a few roans. This site is has great pictures and genetic information.
Icelandics - Icelandic horses are known for their wide variety of colors. This site has pictures of almost every coat color there is.
Thoroughbreds - No, really! Those plain old chestnut, bay and brown horses on the race track? Boring! This site belongs to a farm specializing in breeding colorful Thoroughbreds - worth a peek to see the white horses on the track.
And this is just too much:
Mules - Mules combine the horse and donkey coloration in really wacky ways. You must see the crazy appaloosa patterns on some of these mules. Mules are adorable.