Common Colour?

What is the most common horse colour in the equine history?

Don't know what this has to do with training, but when you're talking 'whole horse history', as domestication is such a tiny percentage of that, I'd say dun - camoflage colouring. As with dogs, rabbits, cats, etc, horses only really got colourful when people bred them that way.

If you're talking about domestic horses in, say, the last 100 years, probably chestnut or bay.