I took a round about approach to setting up my color cues.
I desaturated my volume and pan background .pngs so that the colors chosen with the "Theme development/tweaker > Fader background when automation settings" would not shift in hue.
Then I experimented with the settings until I had three distinct colors to use.
After that I worked on the *_env_* and *global_* .pngs to match the coloring to that which resulted from the "Fader background when automation" color settings.
It would be nice to have control over the overlay mode and alpha so that any color could be used effectively. The pastel colors that result from the default overlay characteristics may not be ideal for all theme designs.