Shoot all the green frogs in the game to unlock the Stealth Camouflage. Alternately, Complete the game with no alerts. A good way to get the Stealth Camouflage with no alerts is, if you do get seen, immediately save the game and enable the In-game reset code to return to the title screen. Reload the game and you will begin at your last location with no alerts. Keep using this process until the end of the game in order to get the Stealth Camouflage on any difficulty setting.