Here is the full run-down on British Army beret colours (remember many units wear forage caps with No.2s, the khaki service dress, and therefore only wear berets with combats):
khaki: Foot Guards, Household Cavalry, Honourable Artillery Company (TA), Princess of Wales's Royal Regt,
Royal Anglian Regt, Green Howards, King's Own Royal Border Regt, Duke of Wellington's Regt
light grey: Royal Scots Dragoon Guards, Queen Alexandria's Royal Army Nursing Corps
brown: King's Royal Hussars
black: Royal Tank Regt
dark (Rifle) green: Light Infantry, Royal Greenjackets, Royal Gurkha Rifles, Royal Irish Regt, Queen's Royal
Hussars
maroon: Parachute Regiment, other troops serving in airborne role
sand (tan): SAS
sky blue: Army Air Corps
cypress (grass) green: Intelligence Corps
scarlet: Royal Military Police
green: Adjutant General's Corps (how this shade of green differs I don't know)
navy blue: all other Army units (except Scottish line infantry regts), Royal Navy, Royal Marines who are not commando-qualified
Commando green: commando-qualified Royal Marines, other commando-qualified troops serving in commando units
RAF (grey) blue: RAF (including RAF Regt, who are the only ones who often wear berets)
I'm sure there are mistakes in there. I'm not even going to try to go into hackles and badge backings!