Fish do indeed perceive color. Every fly fisher knows that or ought to know that. Like humans, the retina of a fish have rods and cones. Cones are used in the day and rods at night. Color vision evolved to help fish identify potential food. In the environment of the fish, the background will either be the bottom, the water itself, or if looking up for food it could be the sky. The bottom is normally tannish olive to green. When looking across the water, the background appears pale silver blue. But if the water is off color due to algae or high water one must take that into consideration as well. Skylight becomes more important at dusk and dawn when it contains more reds.
Thus for opportunistically feeding game fish, flies with bright or contrasting colors and/or a lot of flash will make them stand out against the above mentioned backgrounds. The Mickey Finn, tied with yellow and red, and a silver body is one of the most effective attractor patterns. As for dry fly attractors, the Royal Wulff is still hard to beat, with its red and peacock body and white wings. Black flies, because of their strong silhouette also are easy for fish to spot. Let’s not forget patterns that contain strands of flashabou or other tinsel that reflect light when stripped or while drifting through the current are easy for fish to spot.
The fly fisherman also must remember that color behaves differently in water that it does when seen in the air. Water is denser, and the colors are diffused quicker. Cloudy days where there is less overall light will offer less visibility, and colors will disappear quicker in the depths of the water. And the clarity of water obviously greatly effects this as well. This is important in fly selection because certain colors travel farther in low light than others. Red is the first color to disappear, usually at about 15 feet in clear water, followed by orange and then yellow. Blues and greens are visible to the fish as long as there is light. Yet silver and white will be brighter.
So while the Mickey Finn is obviously a great choice as an attractor fly, it would not be as good a choice in murky water or if fished deep. A better attractor might be a white Woolly Bugger or White Marabou Muddler.
Color is also important to remember when matching the hatch. Since fish use vision as the deciding factor to strike, one’s offering must be the correct color. However, very small differences in hue seem to not be much of a factor as most insects will vary slightly in color as well. But if the intensity of color the artificial fly has can be a factor. If the artificial is more intense than the natural it is more likely to catch fish. Why this seems to work is somewhat a mystery. It is understood that fish see deeper into the ultraviolet range than humans, so perhaps they are just seeing something we don’t. It could also be due to the effect water has on colors. Perhaps we’ll never know, but like many things in fishing, why something works is not as important as just knowing that it does work.
While color is probably not the most important factor in a fish striking a fly. The above considerations are nevertheless a good thing to have in the back of your fly fishing mind.