Berets of the United States Army
U.S. Army Special Forces wear the green beret because of their link to the British Commandos of World War II. The first Ranger unit, commonly known as Darby's Rangers, was formed in Northern Ireland during the summer of 1942. On completion of training at the Commando Training Depot at Achnacarry Castle in Scotland, those Rangers had the right to wear the British Commando green beret, but it was not part of the regulation uniform at the time and was disallowed by the U.S. Army. 
The 10th Special Forces Group (Airborne) had many veterans of World War II and Korea in its ranks when it was formed in 1952. Members of the 10th SFG began to unofficially wear a variety of berets while training, some favoring the red or maroon airborne beret, the black beret, or the green commando beret. In 1953, a beret whose design was based on that of the Canadian Army pattern, and which was rifle-green in color, was chosen for wear by Special Forces units.
Green Lantern Corps
"The Corpse" is an elite, top-secret Black Ops division of the Green Lantern Corps. Members of The Corpse are not restricted by the same rules that regular Lanterns follow, and they perform the darkest, most dangerous missions and are allowed to use lethal force. These members do not use Power Rings; instead, they swallow special coin-like disks that give them all of the powers of the standard ring with a charge that lasts five earth days. These disks produce purple energy instead of the traditional green. Members of the "Corpse" do not wear Green Lantern uniforms, or display the Green Lantern symbol. Instead, they seem to prefer black uniforms that have no symbol.