One of the first things Luke Skywalker does after meeting Rey in Star Wars: The Last Jedi is change his clothes. When Rey finds Luke on top of the cliff at the end of The Force Awakens he's wearing lighter colors, white and beige, but immediately after he tosses his old lightsaber over his shoulder he storms off to his hut and changes into darker robes. It turns out Luke was in the midst of preparing for a ritual, specifically, he was planning to light the first Jedi temple on fire and burn the books with it. However, once Rey arrives and Luke realizes he won't be going through with it, he changes back to his normal robes.
This detail comes from the novelization to Star Wars: The Last Jedi, (via ScreenRant) which not only reveals that Luke was planning to burn the tree, but that he'd actually been planning to do so on several previous occasions, however, something always got in the way and prevented him from doing so. The latest interference comes from Rey. This also explains why we see Luke back in the lighter colored robes later in The Last Jedi, when he makes one more attempt to burn the tree. This time he's stopped again by a visit from Yoda, who lights the tree on fire for him.
Rian Johnson made some changes in his Star Wars movie for no reason beyond the fact that he liked his idea better. He actually moved Kylo Ren's scar because he thought its original location looked "goofy." It's possible that Johnson had Luke Skywalker change clothes simply because he thought Luke should be in a darker outfit. Perhaps he thought that with Luke's slightly darker turn in the movie, it didn't make sense for him to be wearing lighter colors throughout the film. However, it appears that even if that was the case, they still found a story reason to justify the change.
Why Luke Skywalker changed his clothes might not have been at the top of anybody's list of questions after seeing Star Wars: The Last Jedi, but it was still an interesting thing to add to the story that might have made some curious. After storming away from an uninvited guest, most of us probably wouldn't first think that what we wanted to do next was change our clothes. However, Luke had been standing up there, thinking about what it was he wanted to do, likely trying to build up the courage to take the step he felt was necessary. Once he realized it wasn't going to happen, he changed out of his formal clothes, the same we'd change out of a nice suit or dress after discovering the formal event had been canceled.
