From a fan edit perspective it would mean making all Jedi saber blades blue in the PT and making Luke’s the first green one in ROTJ. If you wanted to add an obvious explanation you have to add it to the beginning of ROTJ The film could begin with Luke in his X wing with the familiar orange gas giant of Yavin in frame too. It could in the crawl explain that Luke completed his training with Yoda and that the Jedi master tasked him with one last test before his off screen death. As Luke meditates he hears Ben tell Luke he must face Vader again. Telepathically. When he comes to that point a tiny green crystal is retrieved. This would lead into Vader arriving at the new Death Star. An undoing of the victory of the first film. What the crystal does would be hinted at by the colour of Luke’s blade when he first ignites it and add some degree of sense to Vader’s marvelling at his son’s new weapon. As he had used the force to find the needle in the haystack of salvaged debris.