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.
Tan beret
The sand-coloured beret of the Special Air Service is officially designated the beige beret. The beige beret was worn from 1942 till 1944. In 1944 when the SAS returned to the UK they were forced to adopt the maroon beret of the airborne forces as they became part of that command. When the SAS was re-raised in 1947 as 21st SAS Artist Rifles they again wore the maroon beret. In 1956 however the SAS officially adopted the beige beret again, an attempt was made to match the original sand coloured cloth beret from those in the possession of veterans. This proved impossible to do from existing approved cloth colour stocks held by the British authorities, so, as a compromise and with no authorisation for expenditure on a new colour dye the nearest acceptable colour was selected and approved by an all ranks committee of the Regimental Association. In 1958 all SAS personnel switched from maroon to beige. Personnel attached to the Regiment also wear this beret but with their own badges in accordance with usual British practice.