People often ask me how important I think color is when selecting a fly pattern. The answer is usually mixed: I think fly color is really important when you are trying to match natural insects, and not as important (or, better said, still important but in a different way) when you are trying to earn a bite with an attractor pattern.
I remember talking about this specifically with the late, great Dr. Robert Behnke (“Dr. Trout”) who explained that fish see colors, but differently than the human eye does. The cones in the retina of a trout, for example, are more dialed in on the blue side of the spectrum (especially in the underwater environment), which is why things like purple prince nymphs work really well, even though you can turn over a million river rocks and never find anything that even closely resembles a purple prince nymph. Use blues and purples, etc. on attractor flies (especially patterns for underwater) because they are literally eye-catchers.
Purple dry flies, on the other hand, seem like a gimmick to me. At least I’ve never had much success with them.
Which leads me to the question of streamers, which is what I get asked about most. And the answer there is, I always have four colors in my fly box, no matter what: black, olive, orange (or brown), and white. Chartreuse, purple, and red are also key colors.
I do subscribe to the “dirty water, darker fly” rule of thumb, so that’s when I lean on black patterns. Clear water, a little white streamer is good. Anywhere that crayfish are found, and in tannic water, that orange/rust/brown mix is great. Olive is the all-arounder.
And when you are plain old prospecting, and hoping, gaudy is great. I like them looking like a clown convention. But in that case, size and profile are also key factors.
Of course, where you throw it, how you move it, what depth you fish it, how fast you strip it, etc., are all important ingredients in the streamer mix. More important than color, I’d say. Adjust those things several times before you switch bugs. If your streamer isn’t getting bit, odds are, it isn’t the color of the bug that’s the main problem.