My £0.02:
If you line a fish then it doesn't matter what colour the line is - you'll most likely spook it - the fish shouldn't see your line. Ron correctly states that Goddard and Clarke's book is somewhat of an authority on the subject but to say one colour is better than another is fatuous. Why? Well, a light grey or white line may be OK-ish if the trout sees it in his window against the sky, but will stick out like a sore thumb against a foliage background. The reverse is true of a dark green/olive line: it will stick out when viewed by the fish against the sky but not against foliage... To say one colour is better than another simply fuels our line-buying fetishes (me included

).
I have caught as many fish with a lemon yellow line as I have an ivory line as I have an olive green line as I have a heron grey line. However when I've lined fish by accident with all 3 then they've spooked.
As ever; one small exception. Some perceived wisdom says that where reed cuts happen (I'm thinking chalkstreams primarily) then the fish sometimes get used to thin green things floating on the water (reeds) and maybe, just maybe (but not in my experience) they won't be spooked by green lines on the water. However, all this does is prove that while one colour line may be ideal for one particular circumstance, it won't be for another. To argue that one line colour is better than another is therefore fruitless IMHO.