During a hatch, fish tend to feed on the largest numbers of insects on the water. The term, Selective Trout and book of the same name are based on that concept. It would be nearly impossible to have a perfect imitation to match every possible size, color and form of insect you could find. Pick one specific trait of the insect and run with it . . . at least to start. And to make it more difficult, under different light and water conditions a fly looks differently to the fish.
Flies also look differently to fish on bright day than on cloudy day.
Or in low light of early morning or evening. Add to that the view of a fish through riffles or smooth water. Muddy water following a rain will also alter what the trout sees. C olor might not be the answer. During 'selective feeding' trout key on one of four primary characteristics to investigate the fly.