I think DJTechnor already mentioned this, but you would put red/blue wool on the heads of each player, depending on what team they are on, so you could tell the difference. Then say anyone without a hat is disqualified and block the /hat command so that they can't pick up the opposite coloured wool and put that on their head!
I have to agree that two diamond teams would be much fairer, also there is no way of stopping, say, a diamond player picking up some gold armour and walking in to the gold base like that! It was a nice idea, but that is the whole point of this planning process, to find any flaws that may be hidden in the idea! <object class="emojione" data="
https://resources.enjin.com/1489581540/themes/core/images/emojione/svg/1f642.svg?0" type="image/svg+xml" standby=":)">:)</object>