To be honest I fished it for the first time just a little while ago and I just met my friend there so Im not 100% sure about where we were. I just know the water looked like someone poured chalk in it.
The N F Toutle is colored by Mt. St. Helens ash. The S F runs gin clear during the summer. After 2 or 3 weeks of dry weather, the N F and mainstem are clear enough to fish. Even in that water, a black fly is more visible to a fish than a bright pattern.
I caught fish using purple a leech in the Winter on the South Toutle. I have also been told from the guys at Anglers Workshop in Woodland that white leeches and streamers are great patterns in the summer. I personally have not tried them though. Good luck!