The Keys or Keycards are a type of access authorization item found in different places around Quake II's Single Player levels. You need them to open certain locked DoorsButtons or Computer switches to activate it. They appear in red, blue and green. Red Keys are first introduced in Lost Station, Blue Keys are also introduced in Comm Center, sometimes later, Green Keys are introduced in The Warehouse in The Reckoning. They are then used to open Strogg bases in order. They are necessary to continue progress through the level.