This theme for Tidy Plates uses the Multi-Plate feature to set a custom graphical appearance for different types of units.
Friendly NPCs will have shamrocks, Friendly players have water droplets, and other units will have a heart. The heart for neutral units is scaled smaller than the graphic for hostile units. The casting bar is shaped like a little fireball. Neat, eh?
If you are looking for an example of how to create your own custom plates, this theme is a good place to start. No documentation is available, yet, but I'm working on it.