For the last several years I have been using more parachuts than traditional. I feel they catch more fish, for me at least, and I tie the bodies in many colors. Gray for adams, yellow for yellow sallys, tan / brown / olive for caddis. By doing that combiniation I find I do not have to carry so many fly boxs or flies. No matter what is hatching I will have a color in the parachute style. The way the fly floats can also be adjusted by the way the hackle is tied on. Concave side (dull side) down the fly will float much higher. Convex side (shiney side) down the fly will float much lower in the film.
This is from my experience only. Nothing scientific at all, just watching what happens and what the fish are taking.