Masks might be a good idea for the opener, keep people guessing as to who these invaders are and what they really want. Though I imagine they won't stay on too long once the fighting starts.

As for uniform... most of the factions in Bleach seem to have their own uniform, but each individual tends to modify it to fit their own personality. So maybe if we did go with blue robes as the 'baseline', and then everyone could modify their appearance a bit from that?