This subject depends on to many things to give just one answer. If you are fishing Freestone type of streams then color does not seem to matter as much as it would on slow moving spring creeks. But then again I have had fish on spring creeks take flies with colors that are not right. So I guess you would have to say that it depends on the hatch and how the fish feel at the time you are fishing over them.
I tend to try and stay with the colors that I feel match the fly the best. If a person says that color dosen't matter then they have not fished much in my view. I have watched to many times as fish turned down fly after fly tied the same style and then take a fly with the color just a shade different. Now did the light coming through that color matter? Or was it just the color that the fish wanted? Or was the presentation just right?

In my opinion it just dosen't matter that much if you are fishing an attractor pattern but if you are fishing to a fish that is on a hatch it can matter a lot.

I carry flies tied the same but for the color in a few patterns and I have found at times it can matter to the fish I am fishing to which color I use. It sure dosen't hurt to be prepaired. Ron