Dun and cream are 2 separate genes. Dun is NOT a color, neither is cream. They are modifiers on top of whatever base color/other modifiers the horse carries. So there is no such thing as "plain dun colored" ... it is their color + dun. Black + dun = grulla. Some say dun is a dilution because it does lighten the body color, leaving the head and legs dark. Others say it isn't because it adds dark stripes. Either way the head remains the base color, so do the legs, and then it adds the dark dorsals and leg stripes. Bay + dun is Bay Dun. The head usually stays dark bay colored, the body is lightened to aaalmost a buckskin shade, and then there's the stripes. Sorrel + dun = Red Dun.
Then when you have the cream dilution in there too, you get Buckskin Duns (dunskin) and Palomino Duns (I think some call that dunalino.) I think all the ridiculous terms of blue dun/lobo dun/dunalino simply confuse people, which is why I typicall just say base + modifier. So a red based horse with both cream and dun gets you palomino dun. A black base with both would be smokey black grulla, but since cream barely affects black they tend to look like normal grullas. And a bay base (black + agouti really ) with both cream and dun gets you Buckskin Dun.
Then of course double dilutes can carry it but it will rarely ever be noticable.
Usually it's pretty easy to tell by looking the difference between a bay dun and buckskin dun, bays tend to be more reddish throughout the body with darker heads, but when in doubt test for cream.
I'm in the process of making a web page with all of this info on it, and was making serious progress last week, until my computer with all the info on it crashed 4 times, went to the computer doctor, and still isn't feeling well lol! Once I get the darn thing to load pages without refreshing it 5 times first, I will finish it!