It all depends on PvP situation.
If he is attacked by some players, and he don't have any kind of skull you need open White mode to heal him. Yellow mode if you want heal friend which is not in guild, but remember you will get "red cross"(only with Yellow mode not with White mode) and you wont be able to enter depot.
If he is agressor aggressor or someone opened on him revenge skull you can heal him by only opening Red mode, but remember you will have skull, and if your attacked friend will kill person which opened on him revenge skull you will recive receive frag.