As for the colors involved and sizes:
The duns would require about a size 4 dry fly hook(6 longshank). Most locals
use a #10 4xl streamer hook. The duns have an ivory body, with dark markings
on top with an olive tinge to dark gray wings. Tails are dark brown/grey.
The spinners are nearly white, with a tinge of pale yellow(the eggs?).
Their wings are mottled dark on translucent to clear. Size is huge.
Longshank flies work horribly, raise all fish with a good pattern and most
pop right out of the trout's mouth. A soft extended body fly of about 2
inches long on a #12 hook is about right.
If we get a real dark, overcast day, we may encounter the Dark Green
Drake(Litobrancha Recurvata, formerly a Hex family before reclassification).
Makela and I caught this hatch exactly once, and it was memorable. These
things are REALLY big(make the Coffin Flies appear dainty), and kind of
muscular for a mayfly spinner. They hit you with a distinct WHAP! noise.
Normally a late-night hatcher, so don't plan on encountering them, or bother
tying flies for them......

Tom