It would be best to use variables because armor can receive damage and code will not read it properly. But if you want it to be armor do like this: Player Event Player damages player > If player is wearing [Damager] (red leather helmet) AND IF player is wearing [Victim] (red leather helmet) > Cancel event.
This should work.
To get damager and victim hold shift and right click on sign (if player is wearing).
And make sure you have nested if function. Consider these <> as pistons. Your code should look like:
player event player damages player If player is wearing [damager] > If player is wearing [victim] > Cancel event <<