Sorry, you don't know, but the colors for Mountain Horse breeders are every shade of "Chocolate" imaginable.
If it matches "chestnut" for a TB, it's some shade of chocolate for a MH.
If it matches "bay" for a QH, it's some shade of chocolate for a MH. If it looks "grey" to YOU, it's some shade of chocolate for a MH.
It's kind of an "IN" joke for Mountain Horse owners.
I've always known a chocolate palomino to describe a really sooty palomino. So sooty that they truly are the color of chocolate but retain their flaxen mane and tail.
A true chocolate palomino is a red based horse that carries one cream gene and whatever gene causes the heavy sooty color
There are others, like the Rocky mountain horse that Corporal mentioned and several other breeds, that exhibit a gene called "silver", which acts on a black based horse and turns the body chocolate colored and the mane/tail a very silvery-white.