I was playing around with some materials that have a little zip to them, probably only where enough light penetrates the surface to ‘activate’ them. 

The first fly is a very standard nymph at first: Grizzly hackle fibers for the tail, a tan dubbed body of rabbit fur, but then I used some UV Ice Dub and created a small shroud around the thorax area. The razzle dazzle of that material could/would excite as the nymph would drift/rise near the surface. The material took on an amazing quality with just the available light, no flash.
The Roxy's Rainbow was at one time a favorite Central Oregon stillwater pattern. The abdomen is wound rainbow Krystal Flash, reinforced with fine copper wire. Peacock herl was used for thorax and in and of itself is a phenomenal material.

Sometimes a little flash is alright, but sometimes a lot a flash might just add some excitement beyond that darn matching the nymph…emerger….dry. Live a little.