Clear, monofilament fishing line is the most recognizable type. It is nearly invisible, minimizing the chance that fish will notice it and get spooked before they take the bait. There are, however, a variety of different colors available that anglers find useful under some circumstances.
In most situations, a fisherman wants his line to be as invisible to the fish as possible. If the water is clear, clear fishing line is usually the best choice, but in murky water, a clear filament may actually be somewhat visible to fish. Many fishermen prefer transparent blue or transparent green lines for darker waters.
For night fishing, a visible line can actually be an advantage. Blacklight fishers employ a fluorescent line and an ultraviolet light to increase line visibility. With the light, they can watch their fishing line for signs of a bite and gauge the lengths of their casts without drawing a lot of attention and spooking the fish the way a bright white line would.
In many cases, color is not the most important feature of a fishing line. Gel-spun polyethylene, for example, has a dark gray color that some anglers see as a drawback but is popular for other reasons. The line is very strong and has very little stretch, so many people see the color as only a minor factor. This line is particular popular for deep-sea fishing, where the color will be less visible and the strength and lack of stretch will make more difference.