In fact, the masks are the only clue when it comes to distinguishing friend from foe. The Death Eaters wear black, so do we. What is actually a pretty smart move by the Resistance also means double caution. If it weren't for the masks, you would definitely run the risk of injuring or even killing fighters from your own ranks. I can't help but wonder why the Death Eaters never thought of getting rid of this telltale feature. But I don't have time to ponder this question.