We found this page (via AlfaBB)
ALFA ROMEO PAINT CODES
And took the line:
AR005 GARDENIA WHITE 8382 * * 1963-68, 1973
To be as close an authority as we would get!
Interestingly I also found a reference to a S African 1961 Giuietta in that AR005 colour:
Sprinting About - Alfa Romeo Bulletin Board & Forums
Can't find the link now, but I did read that Gardenia white had more blue/black in it and was less 'yellow' than some whites...