Star Wars: The Rise of Skywalker came packed with surprises, including an unexpected change to the look of Luke and Leia's Force ghosts. After Star Wars: The Force Awakens introduced a new generation of heroes and Star Wars: The Last Jedi marked the end of Luke Skywalker's (Mark Hamill) extended journey as a hermit, The Rise of Skywalker revealed that the nefarious Emperor Palpatine (Ian McDiarmid) has always been pulling the strings of The First Order even after his apparent death and the destruction of the Galactic Empire. Fortunately for the galaxy, Rey (Daisy Ridley) and her friends manage to overturn the tyrant and ruin his plans once and for all. Each movie in Disney's sequel trilogy gave the iconic trio of Luke, Leia (Carrie Fisher), and Han Solo (Harrison Ford) their farewell, and following Star Wars tradition, the Force-sensitive Skywalker siblings appeared in ghost form at the end of the movie to celebrate another victory of the light side.
When Obi-Wan Kenobi (Alec Guinness) meets his end at the hands of Darth Vader (James Earl Jones) in Star Wars: A New Hope, his voice manifests to Luke in the movie's climactic moment, but it's not until The Empire Strikes Back that the Jedi master first appears as a physical embodiment of his own spirit. This ability to become a Force ghost has two main conditions: only Jedi can learn it and only Jedi can see them. However, not every Jedi can become a Force ghost when they die, as the ability isn't at all common and actually takes effort to master. The reason why it has become so prominent is that Qui-Gon Jinn (Liam Neeson) learned it from an obscure group of Jedi Priestesses and passed the knowledge down to Yoda and Obi-Wan, who in turn helped Anakin Skywalker learn it quickly before his spirit vanished forever.
Force ghosts are known for their bright blue tint and their transparent quality, but Luke and Leia's Force ghosts in The Rise of Skywalker look considerably more solid and lack most of their traditional vibrant color. Unlike the vivid appearance that characterizes all previous Force ghosts, which even tend to interact with their physical environment, the Skywalker siblings give the impression of being a portrait rather than "living" spirits. So far, there hasn't been any clear in-story explanation for this, but judging by the context, both Jedi are supposed to be a more subtle apparitions to Rey since their presence is an approving nod to Rey's decision of becoming a Skywalker by choice, thus carrying on with their legacy.
Their faint color is also due to planet Tatooine's yellowish atmosphere. Earlier in the movie, when Rey attempts to burn Luke's lightsaber on the planet Ahch-To, Luke's Force ghost appears and convinces her to keep fighting. His ghost form actually appears to be the most saturated one in the franchise to date. Accordingly, this is due to the planet's weather and atmosphere, as well as the ghost's contrast with the fire coming from the burning TIE fighter and the lighting in the scene. From a narrative perspective, Luke is actively using the Force to persuade Rey to prevail in the way of the Jedi in this moment (even lifting his X-Wing out of the water), while he and Leia are already at peace at the end of the movie, which may explain their softer look.
The Force is perhaps the most abstract element in all of Star Wars. Many of its characteristics tend to change as the saga evolves, including the abilities it grants to its users and how it manifests to Force-sensitive characters. The Rise of Skywalker is the last time fans will ever see Luke and Leia together in live-action, but the appearance of their Force ghosts - along with the Jedi voices at Exegol - can be a hint at a new mystery in future stories. After all, the Star Wars franchise is quite familiar with retconning small inconsistencies into important parts of the lore.