Buckskin = Bay + Cream. Generally no dorsal stripe, but can have a very lookalike stripe, but is called countershading.
Dun = Bay + Dun. Similarly colored to buckskin, but this has the "primitive markings" such as leg barring, shoulder barring, and a distinct dorsal stripe.
Dunskin = Can through you for a loop. Buckskin + Dun. Same color as a buckskin, but does have leg and shoulder barring and a dorsal stripe.
Dorsal stripes look like they were drawn on with a marker. Countershading has blurred lines and isn't distinct.
If you want to know if a horse is a dun or a buckskin, duns will have leg barring. Buckskins will not.