Sadly there's no way to do this on custom map. But it's possible to add some additional mark for red team, for example buff status under HP bar that doesn't add any actual benefit. Not sure if there's such though.
The problem with this event is that the team distribution is sometimes "way too random", which makes teams not balanced. But I will probably change the registration method from random team distribution for each person, to the same that we use for random PvP event (chest digging at Sandsong Village). This should help a bit.