The book doesn't really say. Ahsoka hears the crystals calling her, then after she kills the 6th brother she takes his crystals and puts them in the hilts she made with some other parts of his weapon and when she turns her new sabers on the blades are white. But the book doesn't describe her meditating with them or deliberately doing anything to affect the crystals and their color. Later she tells Bail she restored them, but how? My only guess is that because Ahsoka was resurrected by the Daughter and now is the very embodiment of the Light side that her handling the the crystals which had chosen her was enough to undo what the 6th brother did to them.
