Dun factor is a dilution gene. It dilutes the coat color and adds primitive markings including a dorsal stripe and leg barring.
Dun Factor affects on different base coat colors:
Color of horse resulting color with dun factor added
Sorrel, chestnut 
Red Dun
Black Grullo
Palomino Dunalino (dun factor will appear faded)
Buckskin Dunskin or Yellow Dun (dun factor may appear faded)
Smokey Black Smokey Grullo (dun factor may appear faded)
Amber Champagne Amber Champagne Dun
Sable Champagne Sable Champagne Dun
Classic Champagne Classic Champagne Dun
Gold ChampagneGold Champagne Dun
Dorsal stripe and front leg barring are always* apparent in dun-factored horses. Horses also carrying a cream gene may have a faded appearance to their dun factor markings. Other dun factor markings that may be present are:
Cobwebbing on forehead
Bars over the eyes
Ear tipping or barring
Withers barring
Leg barring on hocks or inside hocks
ventral (belly) stripe
*Some grullo horses can be so dark that they appear black and it is very hard to see their dun factor.
A dun cannot be produced unless at least one parent is a dun.
How to tell countershading from dun factor on a foal: A dun-factored foal will have a dark, prominent stripe down the back that has clear defined edges and runs from mane into the tail. He will also have definite leg barring.
Foals with countershading do not have leg bars and the stripe down the back does not have clear defined edges and does not run from tail to mane (usually stops short of the withers).
Also the dorsal stripe should appear to "run" into the tail with lighter hairs the same color of the coat to either side of the stripe.
