It sounds like you could use the ColorizeVolume module. It creates an rgba volume for you and you can toggle the visibility of individual segments.
https://github.com/Slicer/VTK/pull/43#issuecomment-1752104540
https://github.com/Slicer/VTK/pull/43
https://discourse.slicer.org/t/new-colorize-volume-module/32254