I prefer bright colored backing for a three reasons. In some lighting conditions fluorescent charturese, orange, yellow, or pink backing is easier to see, which can be helpful to a guide or captain maneuvering a boat while I'm fighting a fish. I also like that little highlight of color that shows up in photographs. When multiple anglers are fishing from a boat and are hooked up simultaneously the bright backing colors are easier to sort when we have lines crossing over and under each other. This is a common situation when several anglers get hooked I'd on tuna or other pelagic species.

The fact that some colored backing may tint the back end of the flyline doesn't bother me. It doesn't hurt the line and it's the end away from the fish.