Green and Tan are the two major colour identifiers for opposing teams in a MilSim environment. Black / Blue / Grey etc will get tagged onto what ever team identified by the host. Some games stipulate you must wear a certain colour and that other colours are banned.
Best bet, go buy Woodland and Desert Marpat. These two will ensure you can join either side of a game you want to play at.
Black is rarely used outdoors due to it creating a void in darkness, and lack of blending in nature. But to each their own.
Ár skal rísa, sá er annars vill
fé eða fjör hafa. Sjaldan liggjandi úlfur
lær um getur né sofandi maður sigur.