Many "pickup" or "skirmish" games can get a little confusing sometimes, but can usually be split along what everyone is wearing for camo. Ie: "woodland and multicam on one side, everything digital on another side".
Better organized, "milsim" or "scenario" events are planned out well in advance and here team identity is sorted out well before hand. An Afghan-themed game I attended this season had the "NATO" team in ACU or CADPAT, with the "insurgents" dressed in a mix of camo and civies. It was really easy to tell teams apart.