[Oppugno]

The tapped colour is set/reset every frame while the nameplate is tapped. That is what is causing your flickering bar colour. I know of three 'solutions':

1. Set the bar colour every frame. (CPU expensive)
2. Create your own health bar and display that instead of the provided one.
3. Ignore threat when the nameplate is tapped. (Any mob that doesn't become a shared tag once you engage it likely isn't relevant to you.)

The only parts of UpdatePlate() that need to be run in an OnUpdate script are lines 63-65 so you have a massive overhead calling that entire function.