That is an interesting one. There is "blending mode" i. rig mode if you click on a layer, but i have never used it and it's not a behavior you can record (you are changing the puppet). You could maybe use a trigger to put a transparent black layer in front of things to make it look darker when triggered. Or put a big semi-transparent rectangle in front of the whole scene to make it look darker. Depending on how important, i might clone the puppet (export as puppet then import a s puppet, which makes a new copy of the artwork file) and then edit the copy by hand. Don't rely on Ch - just use photoshop or illustrator to change it.
I think i remember someone once trying things like creating a puppet to represent a ray of light. It was semi-transparent with a different blend mode, so character was inside the ray shape got brighter... i don't remember tif it worked well or not - int3resting idea though