I have an issue with Weak auras that I can't seem to figure out.

I was looking for a warning or an icon to pop up when taking direct damage in PVP to change from my Fel armor to my Demon Armor and another to pop when not taking direct damage for lets say 5 seconds to switch back.

I've been tinkering with this for about an hour and can't seem to find out how. Also looked all over this forum and was unable to find anything related to this and weak auras