This is not actually a bug report but a feature request.

I am just getting started with plater so please excuse me if I am not aware of some of the addons features.

On battlegrounds, displaying the health bars in class colors is fairly helpful for quickly spotting the optimal target; however, quickly finding the warlock among a horde of pets/ghouls/treants can be rather challenging as the class color of a warlock and the standard not-aggro color for npcs are (almost) identical.

It would be nice if it was possible to select an own color for player-summoned npcs such as pets, ghouls and treants because they behave differently than npcs and their threat is not actually that important. (Standard color could be some yellow/orange (high threat color).)
I was surprised that this is not a standard feature. On your Discord, I could find multiple people searching for this feature – WeakAura imports have been suggested as a solution however I could not find such a WeakAura and also I think this should be a standard feature of this addon.

Thank you for the request. May be added, might not be added. No decision on that yet.

Generally: Plater is build around customizations using mods and scripts. Coloring NPCs based on type, etc. is done through Plater mods (not WeakAuras; although both are shared on wago.io). There are some mods around on wago to get started and if you need help modifying those to your needs, there's the #plater-script-mod-help channel in the Plater section.