The sulphur advise is good. Most of my best luck has been with coffin flies after dark. I'm not sure if pattern means that much in the dark and I have had days when a #6 white Wulff worked, my current go to is :
Tail: deer hair, short hook gap length (long tails lower hook up rates for me)
Abdomen: Cream dubbing
Thorax: black dubbing
Wings: Cream or clear Hi-Vis semi-spent
Hackle: white furnace, clipped on bottom
I like a hackle and semi-spent wings so I can see the fly in the dark. Flat wings and no hackle don't show up in the dark for me.
Rarely had luck with duns anywhere, except Fishing Ck. There a Catskill style pattern in very pale olive to pale yellow worked. The duns where hatching like a wobbly colt getting up and dropping back again. Full hackled pattern seemed to work well for that. But generally when the duns are around sulphurs or brown drakes seem to fill the bill.