There are 4 basic Dun Colors.

This is because the dun gene can affect the 
four basic horse colors of red, black, bay or seal brown.  
One dun gene looks the same as two; so without other color genes added, there are four basic colors of dun.

Click on thumbnail photos, below, to see full size.

Common Dun

Also called  zebra dun, dun, regular dun, bay-based dun, and bay dun.  This is Kiger Cougar of Kentucky Horse Park with Jennifer, his handler/rider.

Base color bay, plus dun gene(s).  This basic, or most common, dun is the one that can be confused with a buckskin.  It will be a tan color with a black mane & tail and the following markings which may be anywhere from dark red to black in color: lower legs, a distinct *line* down the center of its back, and zebra-like striping or "barring" on its legs, most often found at the top back of the foreleg. There are many other possible markings and traits that appear on dun horses, but these are the minimum for a normal bay-based dun.  The horse described here is genetically black based with at least one bay and one dun gene.

Red Dun

This mare is Tinker of Carousel QH's.

Base color red (chestnut/sorrel) with dun gene(s).  A red dun will look like a chestnut (sorrel) horse with the body color lightened and the mane, tail, lower legs, dorsal stripe, leg barring, etc. remaining the "base" chestnut color. 

Grulla

Shaila, a 3 YO (in these pics) AQHA/NFQHA Grulla mare - Tested Ee aa, neg cream, neg LWO, and is Dd, with year-round brindle pattern (see pics below). Photo contributed by Tara, www.risingmoonranch.com 

A grulla has a black, non-bay base with one or two dun genes added. The body color tends to be what is commonly called dove-grey, but can vary from nearly black to silver.  It may take on a golden or olive cast if there is a cream gene also present (smoky grulla). The mane, tail and markings described above will be black or a darker shade of the body color. 

It's Spanish, the name of a crane (water bird), and is pronounced GROO - YA.

Seal Brown, or Brown, Dun

"Magic", bred by Lylian Thayne of The Secret Ranch

This color is harder to identify definitively, since there is no test (yet) for the type of Agouti that produces seal brown, to separate it from the one that produces bay.  This mare and foal are good examples of what one would expect seal dun to look like. I'll ask the breeder, Lylian Thayne, if both have been tested for agouti.

Perhaps it's safest to say if you have a horse that looks grulla, especially olive or golden grulla, and tests Aa or AA for Agouti, it may well be a seal dun.

To see how Dun looks in combination with other 
color modifying genes, see "combinations".