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.
Black beret
In the Brazilian auxiliary military forces of the Polícia Militar (Military Police), specially in Minas Gerais state, officers sometimes wears black berets as official parts of patrol gear.
In the Canadian Armed Forces, black berets are worn by Royal Canadian Armoured Corps soldiers and by all sailors (except military police and special operations sailors).
In the Chilean Army, the black beret is worn by the paras and the special forces.
In the Croatian Army black berets are worn by Military police units and by 1st Guard Brigade (Croatia).
In the Czech Army, military policemen wear black berets.
In the Danish Army, the black beret was originally used by all combat regiments, but now it is worn by Jutland Dragoons, Guard Hussars, Royal Life Guard, Army Combat and Fire Support Center, Garnisonskommandant Vordingborg and 1st Danish Artillery Battalion
The Finnish Panssarijääkäri (panzerjaegers) and members of the Armored Brigade also wear black berets.
In the German Army, an oversized black beret was introduced during the National Socialist era for tank crews, to be worn over the crash helmet; however this was dropped in favour of a black garrison cap during World War II. Today the black beret (of conventional size) is worn by the Armoured Corps and the Armoured Reconnaissance Corps.