One possible idea:

The event would be hudhealthbonuspulse.

But then it would only turn green when your target is buffed.

I'm not sure there is a way to do exactly what you want though.

I think what people generally do is move everything up and off the screen, then move just the time value back down onto the screen.

Yeah shame on moth for trying something different...maybe he should just stick to switching around the fonts and colors in garm3n hud. Or better yet spend all his time making the main menu look pretty.