Hey guys, perusing the forum when I saw this and wanted to weigh in... In the human eye, there are two things to detect color and light, these are rods and cones. Rod cells perceive contrast in low light. Cone cells provide color vision. Like your eyes, fish eyes contain both rods and cones. Their eyes are replete with the three chemicals that allow humans to see in a seven-color spectrum, plus a fourth chemical. The fourth chemical, common to most predatory fish, permits them to experience the ultraviolet range. Another fish-eye feature is "eyeshine." Eyeshine helps fish like walleye and deep-sea species to see well despite their dimly lit world. Reflected light bounces off a mirror-like layer near the back of the eye allowing light to pass through the eye twice. (Raccoons and other mammals that favor the night have the same layer in their eyes). It's important to keep in mind that the medium fish see in is denser than air, you need to remember that long wavelength light (red and orange) disappear in the first 15 meters of water. Short wavelength light (blue and ultraviolet) penetrate far deeper. Basically, in that first 15 meters of the water column, color matters. Now that that's out of the way.. take this into consideration: hold an object up to the sun and look at it. It doesn't matter the color, you won't be able to discern it, all you will be able to make out is the silhouette. This is why in dry fly fishing (or any topwater fishing) size and shape come before color. Now if you were fishing streamer patterns, nymphing, crankbaits, or soft plastics in gin clear water on a sunny day, nuances in color absolutely matter. If fishing in murky water or at night, contrast is more important (Choose a dark color like black). Hope this makes sense. Sorry for the long reply, I get excited when I can fuse science and fishing.
One more thing- as far as colored line goes (Cajun Red).. its all bologna, just a marketing ploy. A red monofilament line past 15 meters will actually appear black in color, perhaps this is beneficial because the deeper you go in the water column the darker it gets due to light penetration limitations, but it doesn't just disappear. The important thing to consider when talking about fishing lines is refractive index. The closer an objects refractive index is to that of water, the more invisible it will appear. We could get REALLY in depth with this but just know that the line with the refractive index closest to that of water is fluorocarbon. That being said when fluoro is knicked or frayed it will be very visible (more so than mono or other lines) so you need to be hyper-aware of that factor. Graduated with a degree in Biology and Chemistry this past May.. As you may have gathered, I am applying all that knowledge to fishing.. haha!