Do fish see colors? That's a question I've heard talked about since the day I tied on a fly over 30 years ago. My answer has been no, but the argument can be made that it's yes simply because if the fly your fishing is dark and what's on the water is light chances are your dark fly will be rejected. Now I grant you this is a very simple explanation, and there are so many other things at play that will cause a fly to be rejected.
Below I have tested a few soft-hackles and spiders in a glass of water to try and simulate what a fly looks like in it's emerging stage. I've used various patterns, all different accept for the first two. This little experiment shows some pretty big differences in how a trout looks at a fly.
The first two flies are partridge and orange patterns, the same exact fly. You can see the orange silk body showing through the very sparse dubbed body. Also visible are the tiny air bubbles trapped in the dubbing and along the hackle fibers.
This is the exact fly as the first. Notice when photographed in a different light the fly looks totally different, especially the color. The trapped air bubbles are more pronounced and the body color takes on a different light.
This is a partridge and orange tied without any dubbing. This fly has a slight-sparse thorax and partridge hackle. The trapped air bubble is highly visible.
You can also see a very distinct rib in the body. This is accomplished by winding just the silk thread. It is not clear in the photo but those ribs in the thread catch light an act as another attraction.
In this last photo the fly has a fully dubbed body, it is slightly tapered and has a gold tinsel rib. Naturally the tinsel rib will draw attention but the key is the almost perfect ribbing in the body showing an insects natural segmented body.
A key to all of these flies is the soft-hackle....it's alive.