I think a lot depends on the weather conditions. When I have been diving in bright sunlight, anything above me was dark when viewed against a bright sky and stood out like a sore thumb, even fish with a white/silver belly.
When swiming along the edge of mangroves and looking up with a "treeline" behind, then white/pale things stood out, dark things blended in. Any stick or lraf floating by was very difficult to see.
From that i would draw that a dark line on a narrow tree lined river would perhaps blend in more to a trouts world. However, the disadvantage is that the angler cannot see the line. If the angler cant see it I would suggest neither can the fish.
I always loved the peach colour of the Cortland 444 because in low light it really seemed to "glow" and was easily seen by me.
One last thing, any of you that watch fly fishing videos, have a look at the colour of the lines when seen from underwaqter, a pale line really is obvious whilst a darker line is much less visible and even when seen, look more natural as the flotsam and jetsome around it is also dark.
Me, I use Mikes pale lines...coz I want to see them and by using a long leader I think that puts the fly far enough away from the fly line.
