Usually the green wire is a ground wire.
I just found this information on a reef forum. Where does it hook up inside the ballast? Anything I should look for?I got my loa 65w 6500k at HD. I use it on a 10 gal ref., and I like it. It did require a little simple wiring. Get a grounded drop cord, cut the male end off at whatever length you need, black wire (cord) to black wire (lamp), white wire to white wire, attach the green wire (cord) to lamp housing. You might want to do a search for LOA on this and other boards for the product #, and read about failures some people have had with these lights. I have been using mine for almost 11 months with no problem.
I don't know much about HPS, but I have seen them in the outdoor lighting section at HD