Admittedly I have a tendency to way overanalyze things and this question has been consuming way too much of my limited biological CPU. Here are some possible considerations I’ve mulled around: a) It doesn’t really matter. b) Match the color of the fly when possible. c) View it as an attractor/flash element. d) Think like a bass angler; brass for stained water or overcast days and silver for the reciprocal. e) Choose a bead color that will best suggest the gas bubble given the color of the ascending nymph you are tying (if tying these). Any help/comment very much appreciated.