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.
Uniform beret
Auxiliary police officers of Certis CISCO and Aetos Security Management don dark blue berets when performing escort and other high-risk duties, as do specialist forces of the Singapore Prison Service. In addition, student cadets of uniformed youth organizations such as the National Cadet Corps also wear berets of different colours. National Cadet Corps has three distinct beret colours for each of its services (Land, Sea and Air), these are NCC Green, NCC Black and NCC Blue for each service respectively.National Police Cadet Corps wears their own separate dark blue berets, while National Civil Defence Cadet Corps wears their separate black beret.