A:
There are many different ideas about line color and just about all of them make sense.
Some say that the color red disappears in the water. If so then why the current trend to use red hooks to increase bites.
In my opinion the soundest theory is to match line color to the waters color and type of fish. For up-feeders such as crappie, trout, perch and others using surface lures use blue or blue camo line to blend in with the color of the sky. A blue to white or smoke color blends with the clouds and sky. For down feeders such as bass, catfish, pike and others use a green, or brownish camo line. In crystal clear water use clear line.
It is also said that fluorocarbon line becomes completely invisible in the water. I have some fluorocarbon leader and I see it under the water more than the monofilament lines.
Some use clear line for all water colors. Think about a clear glass that contains Kool-Aid or iced tea. With clear ice cubes in the glass the cubes are very easy to see as they contrast with the liquid drink. If the cubes are the same frozen tea or Kool-Aid then they blend in and are much less visible in the drink. From a short distance one cannot see the cubes in the drink at all. With the clear fishing line having the same effect in various water colors then the clear line would show up like the ice cubes in the drinks.
Suffix Seige Camo is a brown to tan color variation that blends perfectly with the muddy water such as in the Southern Minnesota lakes or the Midwestern rivers and many of the ponds. The Northland brand panfish blue camo line is perfect for up fishing as it is a variation of white to a darker blue color line. The Stren "Walleye" color and the green lines blend in well with the bottom and with the underwater plants and algae.
Many that are night fishing prefer to use a fluorescent color line such as fluorescent gold or yellow colors along with a black light so that they can more easily see their line in the dark. The trouble with a black light is that it attracts insects to it much more than does a white or yellow light.