Line colour and how fish see it.
It is not too surprising that experts believe clear monofilament is one of the most versatile lines you can use if you're looking for something that will be near-invisible to fish in most situations, while heavy braided lines are considered to be the most visible. A heavy braided line is the strongest stuff you can use, but is likely not the best choice for heavily-pressured or skittish fish.
In the end, it all comes down to the type of situation you're faced with while fishing. Some coloured monofilaments, such as yellow (usually high visibility lines) work better in muddy or green-hued water, while giving the angler visual cues as to where and what his line is doing. These lines aid bite detection, and remain pretty invisible under water, but should be avoided when fishing pressured waters where fish are line-wary or skittish.
Red--another easy to see colour for the angler above water--is a controversial colour as some studies show that red objects lose their colour first underwater, while divers claim that objects which lose their colour at depth, actually become black. Added to this, many red hook manufacturers claim that they generate more bites since they mimic blood and are more visible underwater.
Others, like blue, may become more invisible like clear monofilament. But if you're really looking for the ultimate in invisible lines, it seems the new fluorocarbon lines are the way to go, due to their excellent light refraction properties. Touted by line companies as the most invisible line on the market, it apparently has the same light refraction properties as water, making it virtually invisible. Some manufacturers make a pink fluorocarbon line, too, and claim it is better and even harder for fish to detect underwater. Scientific studies and first hand accounts of divers underwater have shown that pink loses its colour at various depths. It works to blend in with the water and becomes clear to the human eye and fish alike.
Green is a good colour for blending into the water. The majority of water we anglers fish has some sort of green tint to it. Like camouflage, green line blends into its surroundings and makes a good choice for anglers looking to keep their line invisible to fish. On the other hand, green may be more visible than clear in very clear water. Overall, green is a good line colour choice for many different situations.
Clear monofilament is a good choice if you are concerned about fish being able to see your line underwater. While the properties of fluorocarbon may make it less visible underneath the surface, clear monofilament works well in all situations.
There is another clear option on the market, and it is typically called clear blue. The line is relatively clear underwater, but the blue tint makes it easier to see above water.
While there are many benefits to using braided line, being undetected by fish is not one of them. Braided line offers excellent strength for a thin diameter, casts extremely far, and does not stretch. These properties make braid an excellent choice for fishing around heavy cover and in dirty or muddy water conditions.
To get the most out of your braid in clear water, use a leader of fluorocarbon joined by a Double-Uni or Alberto knot. This allows anglers to have the best properties of both types of line, strength and invisibility.