Maybe it's superstition, but I've always believed that specie of any given kind tend to school together. Mine are not perfect matches, but if I put a blk/glow flasher down then it'll have a fly with some blk & glow it in. Same with greens - if the main color of the flasher is green then the main color of the fly will be green.
One thing I do know from experience is whatever my flasher/fly combo is, I always put out a clean spoon of near exact color to the flasher on another line somewhere near it. Usually below & beyond.
Tom B.
(LongLine)