Use the adjective dun for something that’s a dusty grey brown color, like a dun cow, or the dun entry rug that used to be white.
The noun and adjective forms of dun all relate to color. It probably comes from German roots, and may be related to the word dusk, as a dun color has a dull quality that you might associate with dusk or fading light. A dun horse is called a Dun. As a verb, to dun means to attempt to collect an overdue bill. You might dun your brother for that $10 he owes you from 3 months ago.