There'll usually be two different scenarios that you'll get into.
1. The teams/sides/forces are split up according to colour when you sign up. Something along the lines of...
"TAN vs. Green" or "Digital vs. Analog" or "NATO vs. Other" or "US vs. Other" etc....
When you sign up for those games you declare what side you'll be on...so the host can get a grasp of what the numbers and size of the forces will be.
At the games where the camo pattern is listed the hosts and every other player is expecting people to be wearing BDU's of that pattern. For example, if the game host says..."GREEN Team (Woodland, DPM, CADPAT, Tigerstripe)"...then you're expected to show in one of those patterns, not simply in green street clothing, not in a custom made green jersey.
If in doubt...ask the game host.
2. It's not really stated up front by the host, etc....or it's a walk on game
- you either sign up or just show up wearing whatever
- teams are figured out on the spot...usually keeping similar colours together (guys in black tend to be the odds and ends and are simply put on one side or the other to balance things out)
Unless the camo's are designated specifically, the "colour" is whatever is predominate...whatever it would be if someone saw you at a glance and picked Tan/Green/Other. If you're wearing a black shirt but have a green chest rig, backpack, gloves, hat, etc...on, you'll probably be seen as green.
Hope that helps