Buckskin (A_CcE_) is bay diluted by the heterozygous effect of the c dilution gene. These horses are yellow, tan, or gold, with black points. Buckskin is similar to dun, but does not have primitive markings.
Like dark brown vs. bay and chestnut vs. sorrel, buckskin vs. dun is a little controversial. Most people define buckskin the way I did above, but some reserve the term "buckskin" for buckskins or duns that are light in shade, whether they have primitive markings or not. But since the two colors are genetically distinct
(A_CCD_E for primitive-marked duns and A_CcddE_ for non-primitive-marked buckskins), there is little point in trying to differentiate them by degree of shading, since that's always subject to interpretation by the human eye.
Further complicating the issue is the existence of two color registries, the American Buckskin Registry Association and the International Buckskin Horse Association. Although buckskin, dun, and grulla are colors, not breeds, you can create a registry for just about anything and define your terms any way you want. ABRA and IBHA call "buckskins" almost any type of dilute except palomino (which has its own set of registries and controversies). ABRA and IBHA will register any buckskin, dun, red dun, or grulla, and if so registered, the horse becomes a Buckskin (capital B) and can be exhibited in Buckskin shows (i.e., shows affiliated with ABRA or IBHA). Calling such a horse a Buckskin does not make the horse buckskin-colored, it just means the horse is registered with one or both of those associations. The horse in the photo above, for example, is a Quarter Horse by breed, but is probably also registered with one or both Buckskin registries, since he's a buckskin
Palomino is a dilution of chestnut by one copy of the c gene (Ccee). Palominos feature a gold, yellow, or tan body, and a white or off-white mane and tail.
Palomino is a popular color among horsemen and lay people alike. It has spawned the creation of a few color registries, chiefly the Palomino Horse Breeders of America. Since palomino requires a heterozygous arrangement of alleles at the C locus (the homozygous dominant CC will be chestnut, and the homozygous recessive cc will be cremello), palominos cannot breed true -- in other words, crossing a palomino with a palomino will only give you a 50% chance of a palomino foal, with a 25% chance of a chestnut and 25% chance of a cremello
