Originally Posted by silver creek
Another fact that confuses newbies is that they assume human vision and trout vision are the same and that the color they see is the same as the color that the trout sees. The lens shape of a trout and a human differ and the color vision have some differences.
For example trout have 4 cones for color vision and humans have three. Trout have an extra cone which reacts to light in the UV range. This cone disappears as the fish ages but is thought to reappear at spawning to help guide the fish to spawning location. The two color receptors of blue and green have the same coverage in humans and fish. But the red receptors in fish can see longer wavelengths of red than humans. So trout can a red that we cannot see.
These differences between trout and humans means that sometimes colors that seem incongruous to us attract trout because the material on the fly reflects a light spectrum that we cannot see that matches the natural. For example a shade of purple such as the purple haze fly may work because it reflects light in the UV spectrum that matches the natural. The color of the natural that to us may not look purple at all but it can reflect that same UV spectrum that is a trigger. I'm not saying that UV is the key of the purple haze but it is one possible explanation.
Remember that when we speak of color it is different that size, shape, and behavior. We can be certain that when we match the size and shape with a fly, we are matching the natural; but that is not true for color because trout see color differently that us.
Fortunately for us, nature varies color in trout foods. So we do not need to be spot on in the color choices we can see. But when we cannot see a color at all such as UV or infrared to come close to a match, it places us at a disadvantage. It may also lead to erroneous conclusions such as purple fly does not match the natural catching fish. We don't know for sure that an off color does not match a natural in a spectrum that is invisible to us and may be a trigger for the fish.
OH SOOOO YES.
The August Issue of Salmon TroutSteelheader (starting on pg 34) has an excellent article entitled "What do Salmon See?" by Richard K. Stoll
Article goes into great depth on colour, eye structure, etc., and etc. The price of the Magazine is peanuts compared to having this information going forward. "Graduate School" education in a few pages of print.