Dun horses of any shade are a dilution of a base color.

Sorrel/Chestnut is a basic color the horse will have red hairs. This group is put into 3 shades 1) Sorrels lighter reds (which can have flaxed mane and tails)
2) Chestnuts where the body color matches the mane and tails
3) Liver chestnuts these horses are a very dark red with dark red mane and tails.
A red dun is a dilution of this gene

Dun is a dilution that affects the black and red color on the body but not on the points of a horse (mane, tail, legs).

Chestnut or Red horses are diluted to a pinkish-red with dark red points = red dun

Dark Bay is diluted to a yellow with black pionts producing a buckskin colored dun or =dun
(lighter bays dilute to produce buckskin horses)

Black is diluted to a mouse-grey color with the black points=grullo horse

Dun horses will have darker points with a dark stripe down the back and may have wither stripes and legs stripes.

Hopefully this helps. There are so many new colors that are being produced and and the industry still has a hard time with just the basic colors, it is hard to keep up.


Just a funny little thought on color:
I know of a reining stallion that is registered as a red roan but he is not he is actually a bay roan and the AQHA never picked up on it and made them change it so when people are looking to produce a certain color from him they are not getting what they expect because he is not registered correctly.