If I am single handed Spey casting it definitely helps me to see where my anchor is so a coloured line is better than clear in that aspect. The pale green Rio Sink 1 shooting head shows up OK but I noticed yesterday that the dark brown sinking poly leader disappeared as soon as it hit the water. I would guess a light olive green wouldn't scare the fish too much but still let the angler see what is going on.
Irvine