I agree that we definitely need the indicator for when we're on the verge of losing aggro (but haven't yet).

I don't think a button to alternate versions would be the answer, particularly not combined with reversing the meanings of the red/green indicators. (Too likely for someone to hit it by accident, not realize they've done so, and then be getting the exact opposite message than it's supposed to relay.)

Better to keep the colors always meaning the same thing (even though that does go against the typical red=bad, green=good pattern when it comes to tanks). I think they should just add a fourth color, like maybe yellow or have both a pale orange and dark orange. Give each of the four colors a consistent meaning regardless of your job, something like:

red: you have solid aggro
orange: you have aggro, but barely
yellow: you don't have aggro, but are on the verge of it
green: you're significantly far from primary aggro

A pattern like that could give both tanks and everyone else the info they need.