Line color is something I harly ever think about anymore. Yes, as Gardener so aptly explained there are some situations where a dark colored line is best, these have been rare in my 46 years of fly fishing. During the 12 years I lived in Montana, I never had a problem hooking fish with bright yellow, chartreuse, hot orange, or white fly lines, including on public spring creeks like Poindexter Slough where there is a large number of people fishing on any given day. I also used bright yellow, hot orange, or chartreuse lines on Montana's Big Horn and Missouri, which can hardly be called uncrowded, with no ill effects from having a too bright line.
I also fished Pennsylvania's public spring creeks like the Letort, Yellow Breeches, Penn's Creek, and Little Lehigh without a problem with bright yellow and hot orange lines, as did my father (heck he and my brother still fish these with bright lines).
I also had two very good friends spend six weeks fishing in New Zealand two different years and they fished with bright yellow lines. Guess what? They had no trouble catching fish, despite being told they needed dark colored lines if they wanted to get fish. Granted, they spent most of their time on the south island; but they had excellent fishing with the bright lines, and they did it without hiring a guide. Perhaps if they spent their time on the more heavily fished north island things would have been different.
These experiences have convinced me that in most situations and on most streams, line color is not of much importance. Yes, there are times and places when it might well be important, but 90% or better of the time most fisherman will not encounter those. That is why I don't pay much attention to fly line color.
Interestingly, the least visible colors underwater were found to be white and silver. Black, medium to dark grey, medium to dark green, and brown were found to be most visible. Yet, I've never seen a white sinking line or sink tip in use on a river. And sinking and sink tip lines are also used with much shorter leaders of 3'-6' compared to the 9'-15' or longer used with floating lines.