As delivered, the 600 class were painted crimson and silver.My understanding was that the 600 scheme was some form of acknowledgement of the role the Commonwealth and Commonwealth Railways played in the SG project, the difference in the scheme being the lack of dark blue lining between the crimson and silver sections.
The colour was nothing like SAR Regal Red, which was effectively maroon.
When repainted in Traffic Yellow by SAR, the waist band was Regal Red.
The AR kits colour was close enough but it was never Regal Red, nor was it ever called that to my knowledge.
The 700 class were Regal Red when built, so I guess someone in SAR had a word to AE Goodwin after the 600s turned up in their strange colour.