It's a very difficult one to answer that one. Think for instance about Friesians. They are all EE black apart from the occasional embarrassing crop out of red. Hafflingers are all flaxen chestnut. Rocky Mountain Horses are predominantly silver dapple in colour but not all. Fjords are all dun but have a varied base colour (brown dun is probably the most common).
If you look at feral or wild horses they tend to be "camouflage" coloured. Often brown with sooty and panagre. This colouring makes them more difficult for predators to see because it sort of makes them like upsidedown shadows. So if you look at non domestic horses you get different results than domestic. You won't see many greys in a feral herd. They'd stand out like a beacon! But look at the tribes who depend on their horses for milk, meat, hunting and hides and they're pretty much all the same; kind of Przewaski like. They are the only breed never to have been domesticated and have therefore never had breeders messing with colour genes.
So, a lot has been done with colour. Like the silver dapple gene. In Rocky Mountain Horses it's the prominent colour but in Welsh it's a rarity. And then there's the Icelandic which carries just about every color possible! And then there's chimeras, which happens when two fraternal embryos fuse together. I think this is the best known image of a chimeras.
https://www.google.co.uk/search?q=chimera+horse&client=tablet-android-pega&prmd=ivsn&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjpjI_U17jJAhWGuhQKHcphAcUQ_AUIBygB#imgrc=aeeAii4L548h6M: