At first it seems that picking a fly fishing line would be easy. Just match the weight of the line to the fly rod. It is a good place to start, but it gets complicated from there.
As is true with the fly rod and reel, the best fly line to use depends on what you will be fishing for, the weight of the fly and overall conditions. This is fly fishing basics 101.
Trout fly fishing lines are made with different weights, tapers, sink rates and colors. Each of these is written in code on the box.
The fly line weight is based on the first 30 feet of line. The American Fishing Tackle Manufacturers Association agreed upon this standard in 1961. This makes it easier for a fly rod and a fly line manufacturer to be on the same page and for you to put together a balanced outfit.
Keep in mind that if the average fly caster were fishing a small stream and had less than 30 feet out past the rod tip, casting would be a little more difficult.
Trout fly fishing uses four different line tapers.
Fly lines are available in different sink rates: floating, sink tip, interchangeable sink tip and full sinking. The sink rate for any given fly fishing line is measured by the inches that it sinks per second.
There are different options on this, but any fly line that hits the water near a trout will probably frighten it anyhow. This is why you need a clear long leader.
The main reason for a certain color fly line is so you can see it. Watch a video on how fly line is made
The backing is the first line to be loaded onto your reel. It is a thin line that averages between 20- and 30-pound test. The backing has two purposes: It adds length to the fly line so if a large fish runs on you, you are ready, and it helps fill up your spool.
The leader is a tapered clear line at the very end of the fly line. More about factory-made tapered leaders. It is made up of either monofilament or fluorocarbon. The tippet is the very end of the leader that is attached to the fly. Learn about tying your own leaders
A floating double taper fly line with an interchangeable sink tip system is your best bet. Loading the rod with one weight heavier than it is designed for is a good idea.
The line color is really up to you.