I agree that, for the most part, line color makes little difference.
I did find that on a rare trip to the Florida Keys for Bone Fish many years ago that line color did make a huge difference. This is a case of super clear, super shallow water and highly pressured fish.
Scientific Anglers made a line called the slime line for just that purpose. It is clear, and intermediate line (sinks just a few inches below the surface) and with a long leader it worked perfect for the Bones. I have used this line for fresh water as well (little stiff for cold weather), but it does not seem to make much difference in fresh water.
From underwater, looking up at a floating line, a light colored line blends into the light colored sky.
From underwater, look to the side or down on a sinking line, a light colored line would stand out like a sore thumb, but a dark line will tend to blend into the dark background.
These are the reasons that lines are colored the way they are. But, as Joe put it, line color sure helps with line control as well. I have a white line that SA made for night fishing for Bass. LOVE IT.
Yes, most of my arsenal of lines are old, so I don't know how many of them are even still made, but I don't replace what is still working.

Too much water, so many fish, too little time.