Re: Color or shape?
Of course, just about any question regarding fly fishing can be answered with the universal answer - "It depends".
Having said that, when it comes to trout fishing, I've been sort of a one fly guy for the last 12 or so years. I fish the same fly pattern about 85% of the time, unless there is a blanket hatch going on, or if I've gone an hour or so without a hit, it's what I use. My home river, where I do the vast majority of my trout fishing is the Muskegon. It a big river that moves at a pretty good clip. It's got a very diverse biology, although the it's probably primnarily a caddis river. For the most part, fish tend not to be feeding selectively. All that being the case, I feel that if I'm fishing a generic fly, and if I can find the right presentation, I can get most fish to strike. I am constantly varying my presentation when I fish, often times within the same cast. I want a fly that looks a little bit like alot of different things. I like a fly that I can fish dry, damp, and at least a bit wet. I want something that I can dead drift, swing, skate, skitter, twitch, etc.
One thing to think about, many forms of trout food don't "dead drift" in a river unless they are literally "dead in the water". My way of thinking is that applying the correct active presentation, implies to the trout that they are indeed looking at something alive.