I made an interesting observation this weekend that I thought I might pass along. Hopefully others with more experience can expand upon it.
I fished Saturday through Monday on the Clinch having excellent luck (nymphing) on Saturday and Monday. I was averaging a hookup every third or fourth drift and at times felt like I was spending more time fighting fish and replacing broken off/ worn out flies than fishing.
Sunday however was overcast and very slow. I could not buy a strike until I switched from a green yarn indicator that I had been using with success on Saturday to white yarn indicator. Upon switching I immediately hooked up with two fish right before having to leave due to rising water.
Yesterday, when returning to the river, I once again had great luck fishing under a white indicator, but decided to test out the green indicator and continued to catch fish with the same amount of success.
It seems to me from these experiments that indicator color and surface color is more visible on overcast days. I'm assuming that on Sunny days, the Sun's direct light backlights the indicator making color less important.