I say keep it green. I mean, sure, you could ask how would he know the other colour was green, but then again you could also ask how he would even know how to construct a lightsaber? Canon tells us that Luke created his lightsaber from spare parts in Obi-Wan's hut on Tatooine, so we can assume that he used one of ben's spare crystals. We can also assume that, apart from perhaps Artoo, Luke was alone in Ben's Hut and therefore had noone to guide him in the saber's construction, which might explain why his was green rather than blue (which, at the time of the OT was the only Jedi (i.e not Sith) saber colour that we had seen. Then of course, the prequels came along and ruined that little bit of imagination, but even so I think Luke having a green saber works. Let's just imagine their are only 3 types of crystal - blue green and red. Luke just used a green one. He probably wouldn't use red becaue vader has a red one, and maybe he just couldn't find a blue crystal that day. As for purple, personally I would change Mace's saber to match the others. It was a dumb idea giving him a purple saber in the first place.

Anyway, depending on how you edit it, isn't it possible that Luke created his lightsaber under the watchful eye of Yoda? Yoda could have told him about the blue and green.

Right now as I write this I'm thinking what an intersting EU novel it would be if someone were to tell the story of Luke creating his own lightsaber, hunting for the neccesary crystal, receiving extra tuition from Yoda, returning to Tatooine to visit Ben's hut, with the hunt for Han going on in the background. Perhaps while he was on Tatooine building his new weapon he overheard some rumours about Han being Jabba's prisoner and that's how they found him - almost as if he was meant to be on Tatooine at that moment, the will of the force.