Dun is a color pattern seen not on ly in equines but in some other animals as well. It is a form of a dilution, but the genetic control causes the body to be lightened (usually black lightens to a slate blue or beige color and red lightens to "Tan" It usually leaves the extremities (points) unaffected - ie the head and legs. The most familiar pattern of dun is a yellow body, with a dark head and black legs. (See Savanne). Dun Factor Markings are closely linked with this coloration. This usually means a dorsal stripe (eel stripe) and leg barring, silvering on the mane, and cobwebbing or shadow striping on the ears, face, and occasionally on the shoulder (in horses, but common in mules).
To add to the confusion between the types of dilutes, there are non-lineback dune, and those with dorsal stripes. Buckskin and dun are often lumped together since they resemble each other, but they are actually caused by two different genes. The Buckskin horse (non-lineback) is caused by the same gene that causes Palomino. Buckskins are diluted bays, palominos dilute chestnuts.
Grullo is the darkest shade of black-based dun, with a slate-blue or "Charcoal" colored body, black head and legs. The lightest shade of black-based dun is "Silver" or cream dun. Dr. Phil Sponenberg's books on Horse Color and Equine Color Genetics go into great detail for the genetic control and exact descriptions of colors.
The basic result of the Dun "dilution" is that black is changed to Grullo (grulla), bay is changed to zebra dun, chestnut to red dun. The number of shades of dun is vast, as are the descriptions for them. Lobo dun, slate grullo, silver grullo, olive dun, coyote dun, zebra dun, golden dun, orange dun, claybank dun, apricot dun, cream dun, lineback isabella. All are names for different SHADES on base colors, with the added dun factor. Dun is a dominant gene, and only one dose is needed of the dun gene to produce dun shaded offspring. Some duns are misclassified, and this is where a dun will "crop out" from two supposed non-dun parents. Remember though, that not all genes are visually expressed on black coats, so a black-looking animal can hide other genetic factors (even a cross and stripe in a black jack, or the palomino gene hidden in a black stallion).
In paining your models, you will want to make the head and legs a darker shade than the body, but still on the same theme. Blue body, black legs =grulla. Fleshy pink body, orange-red legs = apricot or claybank dun. Get a Quarter horse magazine or Paint horse magazine and study the colors, and the subtlety of the markings.
MULES on the other hand, aren't usually subtle in markings. They have overspray (sooty) markings on the neck, multiple shoulder stripes, zebra-type leg barring. Even when a black jack is bred to a palomino mare, you may find expansive dun markings all over the mule. This suggests that even a black jack may have a cross and stripe (typical donkey "dun" markings) but the test to prove this is complicated and will probably never see finality.
