You're spot on to think that a good cast will only show a trout a clear leader and not the fly line itself. But that doesn't factor in false casts. Point being: On flat water, the fly line must be completely muted and either gray, green, or blue. And in lumpy water, such as in the photo, color matters far less, but a greenish-blueish-gray line will never serve you wrong. Because, as we fish most rivers, we fish all kinds of water—flat, roily, and riffled. Gray-green is the number. If I showed you my trout reels, all you'd see would be grays, and greens, and a few tans or sky-blues mixed in there.