Yes, dun is a Scrabble word!
...and is worth 4 points in Scrabble, and 6 points in Words with Friends
dun
Verb
1. Treat cruelly
2. Persistently ask for overdue payment
3. Cure by salting
4. Make a dun color
Adjective
1. Of a dull greyish brown to brownish grey color
Noun
1. Horse of a dull brownish grey color
2. A color or pigment varying around a light grey-brown color
What Scrabble words can I make with the letters in dun?
2 Letter Words