Camouflage (upgrade) is used on some units which they are hidden from the sight of enemy forces when not shooting their guns at them. This was used during most of the games like Tiberium Wars where Nod's Stealth Tanks (being deployed by Kane at the Atlantic Coast in Brazil) are invisible by Camouflage because they are not shooting at GDI or Scrin invaders against the Brotherhood of Nod. In Command & Conquer: Generals, Rebels have used their own Camouflage to remain hidden when not shooting as in Zero Hour, Prince Kassad's Stealth Rebels are camouflaged and also with GLA base defenses which are hidden by Camo Netting on Stinger Sites and Tunnel Networks.
Another way to camouflage your base is using stealth technologies. The Stealth Generator in Tiberian Sun was used by the Brotherhood to get their bases not seen, while in Tiberium Wars at the Amazon Desert, a Disruption Tower was used to clear a Nod base with a research lab underneath it. During the War Against GLA in Egypt, Kassad took his pleasure to using GPS Scramblers and Camo Nettings to cover his two bases plus forces in Cairo and at a stream near the city. In Great World War II, a Camouflaged pillbox was used by Allied Forces in Europe against an almighty Soviet Union from crushing them in Eastern Europe to both Iceland and Great Britain to win the war in an alternate history which Stalin would've died as a Soviet Premier in both campaign storylines.