I think if it’s to solve the monochromatic nature of the scene, it would be better to change the environment or the lightsaber (as some have changed Leia’s saber to purple) than to make a drastic change that causes so many story questions.

Blue lightsabers have been associated with Jedi, but nothing else blue has been. Even blue blaster-fire has been more associated with the clones/stormtroopers. Blue/cold hued settings have been more associated with the villains throughout. Saying blue = good and red = bad seems really simplistic and not at all rooted in anything beyond lightsabers.

As far as newly established things being different, Exegol and Leia’s lightsaber are both new and haven’t been established as something else in prior films. If the monochromatic nature of the scene is an issue, changing the elements that we haven’t seen in so many of the other films might be a better solution.