More than you ever wanted to know about light and fish vision.
Light frequency stays the same and the speed of light is a bit slower in water than in air, therefore wavelength or color of the light does change a bit as light moves from air to water. However, I think that has a relatively small effect.
What has the greatest effect is how deep the various wavelengths or light colors penetrate water.
The doppler effect, or red shift that astronomers see in stars moving away from us, has very little effect at the speed that fish swim toward or away from an object. Colors do not shift for fish just like the color of a car coming toward us does not look any different when that car is going away from us. That car is moving at a greater speed than any fish, and we don’t see any doppler shift because the speed of the car relative to the speed of light is so small that we cannot perceive any wavelength change. So whether a fish moves toward or away from a fly, there is no doppler shift effect.
The fact that fish can see into the UV spectrum has nothing to do with the penetration of light but the ability of their cones to detect into the UV spectrum. If they could not detect UV, it wouldn’t matter if UV were present or not.
What the fish sees is dependent on three factors. One is the depth of light penetration which varies with wavelength - ROY G BIV for Red Blue Yellow Green Blue Indigo Violet. The red end of the spectrum penetrates the least and if I remember correctly is gone by about 5 feet. The other factor is the color of the fly. Obviously, if a fly is green it does not look blue even if blue light may be present. Only the color that is reflected back to the fish can be seen by the fish. The other colors are absorbed and only green is reflected. The final factor is how much the reflected color of the fly gets absorbed by the water between the fly and the fish.
Therefore, the presence of light of a given color is a necessary factor but not a sufficient factor for the fish to see that color in a fly. The color light must be present, the fly must be of that color so that that color gets reflected back to the fish, and the fish must be close enough to the fly for the reflected color to gets back to the fish.
Fish do not have a macula which is the region of our retina that are tightly packed with cones. Their retina are evenly coated with rods and cones. The density of the rods and cones is 14 times less than in a human’s macula so we see 14 time “better” than a fish. A fish needs to get very close to a fly before it can see the detail we can see at a distance.
A trout has a round lens when compared with a human lens which is more disk shaped. Because the lens of a trout must focus light that enters the lens from water, it needs to be more spherical than a human lens which bends light entering from air. This allows the trout eye have a wide depth of field - i.e., the shape of the lens does not need to change much to focus for a specific distance. So to a trout virtually everything is in focus. Perhaps this explains how they can pick some things out even though we are able to see greater detail.
Finally, no one has mentioned flourescent or phosphorescent flies. Flourescence is the ability of a dye to absorb the light energy of one color and emit an entirely different color. Therfore, even if all red light gets absorbed below 5 feet of depth, if we use a flourescent red material, it will absorb blue light energy and emit a red color.
Phosphorescense is the ability if a material to absorb light energy (usually in the UV range) and gradually emit it over time. It differs from flourescent material in that a flourescent fly cannot store the energy and emit it over time. So if we treat a phosphorescent fly with a UV LED light source, we can fish it at night and it will glow under water. I use phosphorescent strike putty at night to locate my flies when fishing the Hex hatch.