Letort:
I didn't realize it at the time, but when I first started fly fishing a Pale Evening Dun was what most folks used for a Sulphur pattern. Since the suburban Philadelphia stocked creeks I fished most often were almost devoid of Sulphurs coincidentally, the first hatch matching Sulphurs I ever tied were for the Letort...
In those days, my go-to was a size 16 thorax pattern (imagine that), tied with bleached racoon for the body, a dyed dun turkey flat wing and because of the Pale Evening Dun, I used light dun hackle.
I stuck with that cream & dun combo and caught plenty of fish everywhere until one night while fishing the Letort, I realized the naturals were more yellowish than cream so I decided to tie future examples with a pale yellow Superfine body, a dun poly wing and pale yellow hackle.
Fortuitously, right after this decision, I hit the Whiting 100 Pack lottery and scored a $#!+load of pale yellow hackle packs at Slate Run Tackle and a few other shops JUST before the hair feather craze for around $6.00 - $8.00 each. 🙂
HOWEVER, I also tie Sulphurs in all white which I use when the light level gets low and I have caught more than a few fish using my Blue Quill imitation during a Sulphur hatch which is the same fly, tied with a reddish-brown Superfine body and darker dun hackle; (they are sometimes easier to see under certain light conditions).
In other words, I honestly don't think it matters what color hackle you use or maybe even what color fly you use....
Because I fish Sulphurs often at the same spot on consecutive days over the same fish, I also tie a parachute version, a Comparadun version and an Iwamasa hybrid, all in size 16.
I do tie a few in smaller sizes, but to be honest, I could count on one hand the number of times a refusal of a size 16 wasn't turned into a landed fish by simply adjusting my presentation or switching to a different STYLE Sulphur of the same size.
Good luck!!