DUN is a "dilution" gene (like Creme, Champagne, Pearl, and Silver Dapples) which modifies (dilutes - think "bleach") but does not fundamentally change the base color.

Dun is a simple dominant, meaning that the color effect is the same whether the individual has one or two Dun genes. Unlike the other dilutions which tend to dilute the entire horse, dun leaves a distinctive pattern, allowing the base coat to show through in a specific pattern that includes a dorsal stripe, horizontal "zebra" stripes on upper legs, shoulder bars, ear tips, cob-webbing on chest and face, and shaded face, just as though the horse were spray-painted with a stencil.

Red dun mustang from the Maverick-Medicine Herd Management Area in Nevada

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Here's how Dun affects the various Base Colors:

BASE COLOR	WITH DUN
RED	Red Dun
BAY	left: "Max," a Sulphur mustang Right: A bay dun mustang from the Maverick-Medicine HMA in Nevada Bay Dun, aka Classic Dun or "Zebra Dun"
BLACK "Grullo or Grulla"	Calico Mountains grullo yearling Grullo mustang in Colorado	Matt Fournier on Steens Vaquero Grullo mustang in Oregon
	Black Dun aka "Grullo" ("Grulla" if female) Other modifiers, such as "Sooty" as well as variations in local soil and forage chemistry, may also be present to create a unique soft "mouse grey"  "hot chocolate," or "olive" color. The darkest grullos are called "Lobo" duns, and the lightest are called "Silver Grullos."
Buckskin "Dunskin"	Granite Range HMA buckskin duns at Palomino Valley BLM Corrals  Lark, Granite Range HMA mustang adopted by Dave & Ginny Freeman of CA
Palomino "Dunalino" Also called "Linebacked Palominos"	Palomino Duns owned by Becky Delaney Seibel
Other Dun Terms & Variations
"Claybank Dun"	This is a common term, but there does not seem to be agreement as to what it is. I've read everything from "The palest form of red dun" to "like a very pale buckskin dun with points that are not quite black" to simply "also called red dun" Termsd like this have hostorical roots, but now that we have a better understanding of genetics, I find that such terms simply add confusion (not to mention a note of snobbery, since many people won't know what youare talking about). I prefer to use simple, clear terms that describe the underlying genetics.
Any other color or color combination can have Dun.
Yes, Pintos can be Dun, too!	Crow (left) and Lewis (right), dun pinto South Steens HMA mustangs
Dun and Silver Dapples	Silver dun owned by Tori Seavey	Tinkerbelle, a silver dapple dun horse rescued by LipizzanLady

Sulphur Springs mustangs at an adoption

There are certain Wild Horse Herd Management Areas within the American West - most notably the Sulphur Springs in southwestern Utah, Kiger/Riddle in Oregon, the Pryor Mountains in Montana, and the Carter Reservoir herd in California/Nevada - that are well-known for their beautiful Dun colored horses. Horses from these areas are not exclusively dun, but well-marked duns occur in significant numbers in these herds, along with the bays, blacks and reds that make up the base colors. Duns also occur in smaller concentrations in most wild herds.

Kiger horses on the range

Carter Reservoir mustangs (photo courtesy of Jona Kalayjian)

ARE DUN HORSES MORE PRIMITIVE, ANCIENT, PURE SPANISH, etc THAN OTHERS?

Dun (along with bay, black, red, roan, the pinto/paint patterns, gray and appaloosa spotting patterns) came to America with the Spanish Conquistadores in the 1500's. Although a wild horse with dun will usually have some "Old Spanish" ancestry (most wil dhorses do), the presence of dun coloring does not of itself make the horse "primitive" or more "Old Spanish" than another non-dun horse.

Dun is valued for its exotic appearance, and, in Mustangs and Quarter Horses, it is indicative of at least some Old Spanish lineage. (While some modern non-Spanish horses, such as the Fjord, are also dun, these are not known to have contributed to mustang gene pools)

Sulphur Springs, Utah, wild horses at an adoption;
Photos: Lesley Neuman

Grullos from the Calico Mtns HMA, an area not identified as being of strong Old Spanish origin (Like most Mustangs of Southeastern Oregon, Northwestern Nevada, and Northeastern California, the Calico Mountains horses' DNA tests show a close relationship to South American horses of known Spanish ancestry and to the "gaited North American Saddle breeds". However, they lack the specific blood markers that are proof of direct Old Spanish lineage.)

Dun's association with being "primitive" or "ancient" is likely because some very old breeds of horses, such as Mongolian horses and the very ancient Przewalski's Horse, and some old breeds of European Draft horses, are duns. There is also the matter of the Portuguese Sorria, which, in terms of its possible contribution to American Mustang populations, I will not touch with a ten foot pole.

Bottom line is - There is nothing primitive about today's modern dun-colored horses.

Grullo Stud Horse from Twin Peaks HMA on BLM Internet Adoption

DUN vs COUNTERSHADING: Some horses appear to have a dorsal stripe, but it either disappears upon maturity, or with seasonal shedding.  This coloring is called called "counter-shading", a function of the "Sooty" gene, and it can be confusing to horse owners. See this link for more: Sooty Foals and Countershading

Bays commonly have a "line-back" which, although it appears to be a true dorsal stripe, is not Dun.