I've never seen a brown floater
well not a pvc one.
I personally, would have problems picking up takes with a dark coloured line. As i would reckon 75% of all the fish i hook are line movement takes. The pale lines make this much easier against a water with lots of reflected trees.
I would imagine on a large water where there is a lot of reflected sky a dark line might be more useful for picking up takes.
Still, i too, think it makes little difference to the fish.
I know there is a school of thought that suggests the refractive properties of water bend some of the colour to form a halo of the colour around the line, but i'm a sceptic on this ???
Sandy