Idk if you ever got past this but there are ways to do it. I use After Effects and Davinci Resolve. In Resolve, you can create Nodes and select specific sections/colors within the image instead of adjusting the color to the entire image. You can find videos on youtube about Resolve and using nodes.
Another way you could do it is in After Effects by first creating an adjustment layer to do your over all color grade. Then create a second adjustment layer and do a “change color to” feature. Select the color you want to change and then mask around just the lightsaber so no other part of your image is affected. You can also go over the lightsaber and rerotoscope it. As far as i know this is always done as a screen layer, so it will interact with the color underneath it. So doing your “change color to” may still be needed depending on what color your changing the lightsaber to.