After cutting the slot in the pipe, I cover the thing with aluminum foil tape (the stuff that's used in sealing heat ducts, not "duct tape"). In a previous design, I used white spray paint that is intended for use on plastics. Facts are facts, that stuff stinks, really bad. Plus, it stains, foil tape doesn't. I do this because I have reflective walls in both of my systems and I want to reflect as much light as possible. Sure, you could leave them green, but green light is of no use in photosynthesis. If there is any unabsorbed light, then I prefer to reflect it around my grow area so that it is absorbed by my other plants.