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.
Guías de Navarra
The Carlist troops had lacked a regular uniform and had used instead the black beret that was already worn in the Basque region. Zumalacárregui gave these troops distinctive red berets, purchased in France, as a type of uniform. At first, the red berets were accepted with joy by Carlist officers, but they then realized that Liberal sharpshooters found the red berets to be a good target. The red berets were thus retired and hidden in a house in Eulate. When he created the Guías de Navarra, Zumalacárregui ordered that these red berets be brought to him and given to this new battalion, which was, in any case, used for the most dangerous missions.