It took me a little while but I finally figured out where to turn on the option to have OpenToonz render a Level’s filter color and transparency which is set at the top each column in the xsheet. This is very useful and I knew it had to be there. It was hiding right there in plain sight at the bottom of the Scene Setttings dialogue box.
This can be especially useful in organizing a Project’s elements so they can be tracked more easily without altering the actual artwork. While not standard usage colors could also signify elements that are incomplete or that need attention by filtering a color onto them (such as red). Need to isolate an element? Make all columns except the one you are working on mostly (or entirely) transparent. Etc. Etc.
In the animation attached everything was drawn in black but each of the three levels was given a different tint.