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Understanding Fly Line Color
Let's get this out of the way right now. During daylight hours, especially on bright days, it is IMPOSSIBLE for a fly line to just "blend in". From the bottom of a river where the fish live, all fly line looks the same when they look upward - a long, black line snaking across the top of the water. From the bottom of the river, the color of a fly line is immaterial - it instead is just a solid, unnatural thing that is lying on top of the water.
Because of this, the color of a fly line for general daylight use does not matter. The fish will see them all. As such, when selecting a fly line color - get a color that YOU, repeat YOU, can see easily. Take your choice, green, yellow, orange, red, pink - whatever. Pick the line that you can most easily see in the type of fishing conditions you find yourself in.
Now, during very low light conditions - particularly if you fly fish at night for bass and such - fly line color can indeed be important. Oddly, pure black isn't the best color - few things in life are ever "pure black." Instead, the consensus is for varioius shades of dark brown/dary grey - depending largely on the color and clarity of the water where you'll be fishing.