Regarding the old logic of bright line appearing dark against the sky from a fish-eye perspective, this only applies when the line is within the fish's circular window of sight outside the water. You must bare in mind that not the entirety of your floating line floats, some will of course be sitting in the water (something like 1/3 - 2/3rds of the radius of your line), also the dry fly will make a messy depression on the water surface (-much the same way fish are alerted to a floating leader) if not also breaking partially through.
So regardless of whether or not your line and your fly lies with the fish's window it will still indeed be visible to the fish, this has been backed up by my own experience many times by casting to the side of fish that are very close to the surface and certainly would not be able to see my fly through its window, yet the fish still approaches and rises (or gives you the equivalent of two fingers and turns its tail!).
Now consider what a bright, flourescent green flyline looks like against the water surface where the outside world cannot be seen. What occurs here is simply total internal reflection (all the light from the underwater is reflected back underwater) this means all the fish are able to see is a reflection of the lakebed and so the surface of the water will be not much brighter than this.
Obviously the portion of your flyline that is sunk will be visible to fish that will also be able to have see the depression on the water surface left by your fly, many of these fish will probably not notice this tiny depression, but for certain there will be a select few that would notice the appertising insect and would attack it. However many of these fish will also notice the huge line of flourescent green or yellow or orange pointing straight towards the nice morsel of food, and will ignore it like any self respecting trout should.
This means using a dark floating line will on some days offer a slight advantage to the usual neon-dayglow-circus floating lines we usually use, it definitely does not mean that using such flamboyant lines will stop you catching fish.