Why Was Lukes Lightsaber Blue in the Last Jedi
In my opinion the climax of The Last Jedi is not just the best moment from the sequel trilogy but among the best moments in Star Wars, period. We see the return of Jedi Master Luke Skywalker, confronting his nephew Kylo Ren and allowing the Resistance to escape.
Of course this was all through a masterful display of Force projection, and one of the questions people have had is about the lightsaber choice Luke uses in that projection. Luke uses the iconic blue Skywalker lightsaber instead of his own green one (though we have an idea of where that one is), but why?
There are two very important reasons.
1. A Message to Kylo Ren
There are several signs in Luke's appearance that he's not physically there, including the fact that he doesn't leave any footprints. But there's more to it than that, as Luke shows up looking younger – the way Ben Solo would have remembered him. Additionally, Luke shows up holding the lightsaber that would mean the most to Ben; in both The Force Awakens and The Last Jedi, Kylo Ren attempted to get control of this family lightsaber but failed each time. So Luke shows up holding that very same saber.
"[Luke] is basically tailoring this projection to have maximum effect on Kylo," director Rian Johnson told IGN when explaining the decision. "He knows that Kylo's Achilles heel is his rage, and so that's why he kind of makes himself look younger, the way Kylo would've last seen him in their confrontation at the temple, and that's why he decided to bring Kylo's grandfather's lightsaber down there — the lightsaber that Kylo screamed at Rey, 'that's mine, that belongs to me.'"
We can debate whether Luke's decision was made to taunt Ben or to attempt to appeal to him, but regardless, the intention was to send a message to his former apprentice.
Besides, Luke couldn't have known that the iconic lightsaber had been destroyed earlier in the film – but then again, neither did Kylo Ren. As Johnson noted in that IGN interview, though the audience sees the saber split in two, all Kylo sees is a big blinding flash. He's then knocked unconscious, and when he awakes, Rey – and the lightsaber – is gone. But even then, upon seeing his old master, Kylo Ren is so blinded by hatred that he doesn't pay attention to any of the signs and is intent only on killing his uncle.
2. A Message to the Audience
Thematically, Luke's use of the legacy lightsaber sends a message that is well-placed within the context of this film. In fact, in many ways Skywalker's journey in The Last Jedi can be told by his relationship with this saber.
At the beginning, Rey offers Luke the lightsaber. He takes it, looks at it carefully, and then tosses it away before walking off. His past had caught up to him, but he wanted nothing to do with it. All he wanted was to toss the lightsaber away, lock himself in the hut, and live out the remainder of his years in solitude and exile on Ahch-To. But as the film goes on, Luke reluctantly agrees to train Rey and opens himself back up to the Force. Everything reaches a boiling point when Luke finds Rey talking with Ben Solo, touching hands. Luke had been about to join Rey but instead fights her. At the end of it, Rey extends the lightsaber out to Luke once again, offering him the chance to take it. Luke seems dismayed but declines, leaving Rey to conclude that Ben is their last hope. She leaves Luke alone on the island, like he wanted all along.
But then Yoda shows up in Luke's moment of need, teaching him and pushing him over the edge. Luke would hide no longer. So he shows up on Crait, reuniting with Leia and heroically marching out to face the whole First Order with a laser sword. And he's using the Skywalker lightsaber.
At the beginning of the film, Luke tossed the lightsaber away, wanting nothing to do with it. In the middle of the film, Luke declined to take the lightsaber, opting not to get involved. But at the end of the film, Luke picks up the lightsaber and saves the day. It's a message within the theme of the movie that Luke Skywalker has finally embraced the call of the Force to pick up the lightsaber and be the heroic Jedi Master we all knew was inside of him all along.
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Source: https://starwarsthoughts.com/2020/07/27/why-did-luke-skywalker-use-the-legacy-lightsaber-at-the-end-of-the-last-jedi/