My tank is also 6 feet ling and the only tape ob there is holding my 2 pieces of foil together. It isn't that bright but it does reflect somewhat a mirror image of anything close enough. I really like it. I'm using the less reflective side of the foil.
What about a light grey? I have never seen it but I can imagine the fish's colours looking awesome against it, especially with black sand, what fish are going into it btw? I think white will look good but like said above it will be a lot of maintenance.