In general you want to stick to brightly colored line. On most rivers line color does not matter to the trout but does make a difference to the angler. I guide Montana fishing trips
and find that it is much easier for our clients to manage their line when they can see it on the water. If you can't see the line you don't know when to mend. The only time I recommend drab colored lines is when you are fishing gin clear water where the fish are ultra spooky. I use drab lines on our local Montana spring creeks and also used them when I was in New Zealand.