(I just remembered reading somewhere the explanation of Luke's green-coloured lightsabre aboard the Falcon. Apparently, the lightsabre is set to "training mode" - presumably to stop him from accidently slicing off parts of his own anatomy during his practice sessions.)
I actually assumed that as a kid. It just seemed to make sense to me. "Why else would he be waving that thing around in a space ship?", I used to think to myself.