For wartime use the appropriate colour references were prefixed with K3, from the specification number. The colours were those which applied to anything in the RAAF, not just aircraft. For example, Sky Blue was K3/195. There’s no suggestion of any parallel alphabetical code. My source is the 1999 (first revision) Red Roo publication “Understanding RAAF Colours” by Peter Malone and Gary Byk, which in turn references the BALM document “Synopsis of Aircraft Finishes and Sundry Paint Products” March 1945.

I assume that postwar this was replaced by specification K4, but that’s a guess.

Ian Baker’s series of booklet is indeed superb, but you will find it difficult (and expensive) to obtain a set.