Flash text on player's screen

[wass24 0]

I have a unit that puts on and takes off a mask through an addaction command. I want to flash text repeatedly on their screen that says "Mask On" when they have the mask on and when they take it off the text will go away.

I'm guessing that this command would execute at the end of the script that puts the mask on, but I don't know how to write it so it repeats until I take the mask off. This will also be used in multiplayer.

Thanks.