Escape from Valo, the first middle grade novel in Phase III of The High Republic, adds fascinating new lore to lightsabers and how their colors can be temporarily changed.
Warning: This article contains minor spoilers from Star Wars: Escape from Valo by Daniel José Older and Alyssa Wong.
The novel opens with a prologue in which Jedi Padawan Ram Jomaram sabotages the Nihil's Dark Fair on the planet Valo. During his sabotage, Ram disguises himself as a hooded figure in a black cloak, wielding a red lightsaber.
Ram was trying to scare the Nihil into thinking he was a Sith, though interestingly, these Nihil don't even know what a Sith is, as the Sith are believed to have been extinct for centuries and have been forgotten by most of the galaxy. At the very least, the deception prevents the Nihil commander from realizing that the hooded vigilante is a Jedi, as Jedi are known to not wield red lightsabers.
As for how a Jedi with a yellow lightsaber was able to temporarily make his lightsaber red, the answer is that Ram is a mechanical genius, creating a device that temporarily made his lightsaber red. Read the explanation from the novel below:
"He fiddled with the small metal device attached to the barrel until it popped of. It was something he'd invented: an octagonal ring with a pane of red crystal from a broken temple window. The energy-transparent material acted like a holofilter, coloring the lightsaber's blade. When Ram pointed his lightsaber at the ceiling and it powered on, the blade shone its true color: yellow, not Sith red."
This is not the first time that recent Star Wars stories have dealt with lightsabers changing colors. The comics have shown Darth Vader and Kylo Ren bleeding their kyber crystals to permanently turn their lightsabers red as they embraced the dark side of the Force.
The Ahsoka novel by E.K. Johnston went in the opposite direction as Ahsoka Tano used the Force to purify the red kyber crystals in the Sixth Brother's lightsabers and turned them white, which is why she has white lightsabers in Star Wars Rebels, The Mandalorian, and the Ahoska series. Star Wars: Visions isn't canon, but it showed lightsaber colors changing depending on who was holding the lightsaber in the season 1 episode "The Ninth Jedi."
While these changes were achieved through the Force, Ram's temporary lightsaber change is achieved through technology. It is an impressive accomplishment and one that is fitting for a character who has always been mechanically savvy and who even views and connects to the Force through a mechanical perspective.
Later in Escape from Valo, Ram gives this same technology to the three Jedi younglings he teams up with, Gavi, Tep Tep, and Kildo. This helps them also hide their identities as Jedi, and works well with the name of their resistance group, known as the Scarlet Skulls.
It will be interesting to see if Ram's holofilter technology returns as The High Republic continues, or if it pops again in another story further down the timeline. Regardless, it is a compelling addition to lightsaber lore.