From the ABRA site:
BUCKSKIN: Body coat some shade of tan, from very light (creme) to very dark (bronze). Points (mane, tail, legs and ear frames) are black or dark brown. Dorsal not required.
DUN: Body coat some shade of tan, from very light (creme) to a dull or smutty brown (earth tone). Points, dorsal stripe and other dun factor markings are dirty black or smutty brown. There are many shades and variations in the dun color. Dorsal stripe required. Note: The buckskin colored horse with dun factor (dorsal stripe, leg barring, ear frames, shoulder stripes, face masking and cobwebbing) is the ideal color that ABRA was founded to preserve over thirty years ago.