This is from the Sandman database. These are the colours not recorded in any model (body style and luxury level) for the series (HQ, HJ etc):

HJ colours (pre 5/76 colour update):

Cyclamen (1897)
Durham Beige (1889)
Nutria (1893)
Sepia (1902) - have heard of a 253 van getting around

HX colours (includes HJ post 5/76):

HJ in Sierra Tan (1919)
HJ in Royal Plum (1918)
HJ in Berry Nice (probably the rarest of all)

HZ colours (pre 11/78 update):

Antelope (1894)
Dynasty Gold (1936)
Persian Sand MKII (1925)
Tuxedo Black (2214)
Ultra Blue (1932)

Commodore colours (post 11/78):

Ashley Grey (1948)
Dark Carmine (1953)
Mojave Beige (1949)
Panama Green (1931)
Reef Green (1951) - not available on Sandman so may not exist at all
Sandlewood (1945)
Tuxedo Black MkII (1959)