Is there an event for on_healing? Or is it on_damage but a negative amount? If so, you could efficiently add healing as well, in green (or blue for structure repair). Useful for worms and spawners, mosd that add auto-repair or bot repair.
Secondly, since red is damage, the negative isn't needed. I can see it being a config option for those that are colorblind; they may want to see the negative value as damage.
I added configuration options for the color of the text as well as made the text itself configurable. You can even have it display the type of damage but by default it's disabled since in my opinion it led to too much clutter.
There isn't an event for healing, just for damage, and even though fish technically heal you by dealing negative damage it isn't caught by the on_entity_damaged event. There is an on_player_repaired_entity but not one for robots repairing (and it doesn't report the change in health). A work-around would be to track the health of every entity and display changes, but that adds a lot of overhead. I'll look in to doing that for select entities though (like players).
