The beige beret has its beginnings with the early SAS in 1942; they experimented with white berets at first in the style of the Free French Ie CCP (Compagnie de Chaussers Parachutistes), but switched to beige after they discovered the white beret provoked fights among other units in Cairo (apparently the ANZACS in particular).
When the SAS was reorganized in early 1944 into the SAS Brigade, they were ordered to lose the beige beret and adopt the same maroon/berry beret of the airborne forces. As Kev says, though, this was apparently ignored in some circles among the "old salts" ... in particular Paddy Mayne.
The beige beret was officially re-instituted and the maroon beret dropped for SAS in 1957.
HTH,
Scott.