Arriving on the planet Christophsis, he encountered a woman named Gretta, who took him to Dr. Cuata. While Cuata worked on Skywalker's lightsaber, he gave Luke a blank crystal, which after receiving a vision of Yoda, turned green.
The in universe reason is because it was a different lightsaber. He started with Anakin's saber, which was blue. He lost that in his fight with Darth Vader on Bespin, and built a new one by the time he was back on Tatooine, that one happened to be green.
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Lucas and his team gave Luke a green lightsaber because it was easier to see it during broad daylight while filming the barge scene.
Others who possessed lightsabers collected them as a trophies from fallen Jedi. Due to this, Luke was forced to fabricate his own green synthetic lightsaber crystal, utilizing a small blast furnace and a book of instructions found within the home of his fallen Master on Tatooine.
After losing his father's lightsaber on Cloud City, Luke Skywalker constructed a replacement with a green plasma blade, its hilt similar to that of Obi-Wan Kenobi's weapon.
In the aftermath of the Battle of Exegol, Rey buried the famed lightsaber alongside Leia Organa's lightsaber, on Tatooine, the desert homeworld of Anakin and Luke.
The Empire Strikes Back showed Luke losing Anakin's infamous lightsaber during his fight with Vader. So, the bourgeoning Jedi needed a new weapon. He went back to Obi-Wan's hut on Tatooine, and there, he found designs for his green lightsaber.
This can be seen in the novel starwars.wikia.com/wiki/Rogue_Planet. It's because of his mastery of sorsue (sorry for my bad spelling.) Well if you want the ACTUAL reason it because in the original 3 films the Green Light Sabre didn't exist until after Obi Wans death.
We learned in The Force Awakens: The Visual Dictionary that the now-infamous red lightsaber had a cracked kyber crystal. Thanks to The Rise of Kylo Ren #4, we now know that the broken crystal is a result of the bleeding process.
A crystal embodies the Jedi as the heart of the lightsaber, and pure kyber has no color until it's bonded to the Force user. This process likely resulted in Rey Skywalker's lightsaber being yellow.
He gave it to Luke Skywalker, who lost it when Darth Vader struck off his son's hand in Cloud City. The lightsaber then became part of Maz Kanata's collection of Jedi curios, where it called to the scavenger Rey. She wielded it in defeating Kylo Ren, then brought it to Ahch-To and offered it back to Luke.
Leia's lightsaber was a blue-bladed lightsaber used by Princess Leia Organa during her time as a Jedi Padawan training under her brother Jedi Knight Luke Skywalker.
Without a doubt, the most notable Jedi with a purple saber is Mace Windu, portrayed in the prequel trilogy by Samuel L. Jackson. Audiences first got a glimpse of the unusual blade in Episode II: Attack of the Clones, making Windu the coolest Jedi in one cinematic moment.
Relatively recently, Star Wars canon established that the reason a red lightsaber is red is because the crystal bleeds after the user does something really, really bad. Meaning, your lightsaber can change colors, and Rey's has changed colors because she's done something bad enough to cause the crystal to bleed.
Even though Kylo Ren uses a red lightsaber, harnesses the dark side of the Force, and wears a cape, he is not a Sith. Because of that, he can never hold the title of Darth. Instead, he was just a chaotic mess of the dark side (until he eventually found his way back to the light).
The lightsaber's redesign was a byproduct of the bleeding process that damaged its kyber crystal, rendering it dangerously flawed and unstable, and necessitating the lateral vents were meant to prevent the cracked kyber crystal from overloading.
Yoda's lightsaber was one of the personal lightsabers of Yoda, the last Grand Master of the Jedi High Council. In keeping with its owner's stature, the green-bladed weapon had a shorter hilt and shorter blade, making it a shoto.
Yoda's lightsaber was smaller than a standard Jedi lightsaber, befitting his shorter stature. But its green plasma blade was every bit as deadly in combat.
Luke lost his dad's blue saber in Cloud City. He built a green one for himself. Somehow Max Kanata managed to recover his dad's blue light saber before the events of The Force Awakens. This is the blue saber Rey took.
Yoda in his self-imposed exile, decided not to wield a lightsaber anymore. Skywalker offers the lightsaber to Grogu. After Yoda's death in 4 ABY, the lightsaber with its accompanying storage box ended up in the possession of Yoda's last Jedi apprentice, Luke Skywalker.
Rey's lightsaber was a yellow-bladed lightsaber that was constructed by the Jedi Rey Skywalker following the First Order-Resistance War.
Encased in a distinctive silver-and-copper hilt, Leia's lightsaber used a naturally-occurring blue kyber crystal, much like Luke's original saber that he had inherited from his father by way of Obi-Wan Kenobi.
Kylo Ren's lightsaber form was something of a hybrid, which blended three styles together: Djem So, Juyo, and Niman.
Luke Skywalker's lightsaber was changed from blue to green when they were filming because it was difficult to see. This became apparent during act one of Return of the Jedi when the battle on Jabba the Hutt's sail barge was shot during the day. The blue lightsaber blended in with the clear blue sky.