First, when fish are keyed on a hatch of natural insects, you need to throw a fly that looks as close to natural as possible (duh). That's size, profile and color. If trout aren't keyed on a certain bug, the real value of an attractor dry fly is its profile. Throw something big and ugly, and they might just take a whack at it. But they aren't going to think, "Gee, that's purple, I think I should eat that."