What is the difference between dun and buckskin horse coats?

There is a pony at my barn who looks exactly like the pictures of Lundy ponies I see (I was never sure what breed he was). At my barn we always call him a buckskin, but the sites I looked at about Lundy ponies said they can be dun (and other colors), but not buckskin. What is the difference? Is buckskin splotchier, or what?