I would say that brightly colored lines are really useful for anyone who is learning to cast or learning to improve their casting as they are easier to see in the air than the more subtle colors. I don't think there are too many freshwater situations where line color is a big issue, although I have heard that the trout in New Zealand don't like bright lines. A friend of mine was down there in the mid 90s and he took along a Wulff Triangle Taper. It was white and he couldn't catch fish. A guide he met said "dye it grey" so he did and suddenly he was getting fish. When I was first starting out I went to the Montana spring creeks and did fine (well, fine for a guy who was just starting out) with a bright yellow line, so I guess it depends on where you are. In practice when you are casting to a fish you shouldn't be rolling the line over them anyways, just the leader on the final delivery cast. Any false casting should be done so that the line is not passing directly over the fish. Make your false casts off to the side of the fish to measure the length, then make your delivery cast so that you show the fish leader only. Jack Dennis has a good video out that shows his approach to dry fly fishing and false casting over rising fish. I forget what it is called but your flyshop my have one in their lending library.
Loop Canada Pro Staff
FFF certified casting instructor