I think you catch more fish with the dark lines. I notice how well you can see someone casting a bright line through the air over the water and that cant be good. I dont want anyone seeing me including the fish. I use a dull light green for nymphing so I can see the drifts very well and Im targeting fish on the bottom. I use a dark green or a mustard line for anything looking up.

I used to use light gray floating and dark gray sink but the lines I like dont come in that color anymore so thats another issue. It use to be simple.