I have also had some experiences of the type that Sinktip spoke about. The best example was a Thanksgiving Day on an Olympic Peninsula river that had fresh winter fish in it. The water had that wonderful green tint to it that seems to turn on winter fish after a good rain. I fished through a favorite run with a black spey fly without a touch. Then fished through it agian with a Lady Caroline. Next I fished it with a Glasso Polar Shrimp spey. Finally, I put on Glasso's Orange Heron and hit a fish on the first cast. Then proceded to hit 5 more fish in the same run in the next hour. Before this experience, I would have said that color doesn't matter; however, after that experience, I have seen this other times on other rivers.
You did hit on something very important though. Changing to a radically different fly in both size and color works many times with supposedly stale fish. Go up or down 3 or 4 sizes and make a radical color change in the new size and many times a fish will hit. Interestingly, this making of a radical change in size and color of fly was something that Kelson wrote about in his book "The Salmon Fly" and it was written in 1895. Traherne also wrote about it in his book "Salmon Fishing with the Fly" from the same period. Maybe we could all benefit from what the old masters wrote about fishing technique.