Jordie is right, dun won't "hide" on anything except a grey. Also, if he's a curly x Suffolk Punch, he's certainly not getting any dun from the Suffolk side. I believe they only (mostly?) chestnut. Curlies may have dun, but I'd be surprised if Suffolk Punches did.

Honestly, that horse just looks chestnut with flaxen to me. I'm not even sure where the owner is seeing palomino.