I always thought that a dun had to have the cream gene and the dun gene to exist, thus making the whole thing 'dunskin' an oxymoron. Could someone please show me a picture of a dun that does not have a cream gene?? Because theoretically if that were true there would be A LOT of 'bays' running around with dorsal stripes, barring, and cob webbing.
Dun dilutes the coat colour, hence why they can appear lighter. Bay Duns are the type that most people picture when they imagine a Dun horse, however their colouring can range from very light (like the ones confused with buckskins) to darker (appearing almost as a normal bay with dun markings). These 2 horses are both bay + dun:
She looks bay to me. A lot of horses have "false" countershading even though they're not dun, so a light dorsal stripe on a bay horse isn't abnormal. It would be very rare for a bay dun to stay as dark as she is.