I only fish it in winter/spring on clear days when there is some color in the water and sunshine or those bright, overcast days that are so common here in western WA help to light up the flourescent Krystal Flash. Also, I especially like to fish it when the rivers are dropping and clearing after a storm has raised them and dirtied them up.
I only use it in large sizes, #1/0 and bigger. It is fished with a sinktip of appropriate sink rate as dictated by the water depth and speed on a standard downstream swing.
I find the ideal water type to be the throat and tailout of holes or those walking-speed runs that are 3 to 5 feet deep.
The extra flash the cerise Krystal Flash body produces helps attract fish and gets them to move toward the fly.