I'm not sure either. When I first outfitted my pram, I used yellow propylene, then braided white curtain cord, and finally 150 lb clear commercial mono trolling line. I have stuck with the mono. I have to wear gloves to haul line, but that isn't a problem. I caught fish under all three anchor systems, but I like the mono when fishing really shallow bottom contours. When my son was younger, he went over the side for a swim and a look and he said he couldn't see the mono.
I honestly haven't noticed any difference no matter the colour I've used. I've tried yellow ropes, typical white bow lines, black, even 1/8" winch cable in a pinch. Really I think the fish are likely to notice the 8-15' floating thing blocking out the sun shine as much as they will an anchor line. If your one to worry about it I guess every little bit helps.