I used the Channel Mixer effect to make the lightsaber white and the rest black, I duplicated the clip with this effect several times, blurring it twice as much each time, and I set their blend mode to Screen to add bloom to the lightsaber.
I nested all these clips together to make a clip that I can use as a mask (since the mask isn’t perfect, I had to manually mask some elements to make local adjustments). I duplicated the original clip and adjusted its colors to make the lightsaber purple (even if it affected the rest of the shot), then I applied the mask to it.