RAL paint codes are the European standards, and of course GELB is German for Yellow (or literally Gold). Signal Yellow would therefore be a NATO specification, used on British Army vehicles operating within Europe. RAL codes were not used in Australia in the 60's.
Australia has traditionally used British Standards for paints, and the colours are listed in BS381c. This was closely copied when the Australian Standard AS2700 was developed in the 80s.
In WW2 and the 50's, and especially in the 60's when the Series 2/2A were being built, the yellow specified for Bridge Weight Signs was BS381c No356 Golden Yellow. Although the Australian Standard was developed by closely copying the British Standard, there was no exact equivalent of No356 Golden Yellow. The closest AS2700 colour is Y13 Vivid Yellow (which in a funny circular way is listed as an equivalent to RAL1003!), but neither ar the same as No 356 Golden Yellow. This is still available today from paint suppliers by asking for BS381c No 356 Golden Yellow, or AS2700 Y13 Vivid Yellow.
Of course, if any old yellow will do and there's no need to get into the specifics of the accurate paint Standards, then Killrust "Sungold" is available from your local hardware shop premixed and is damn well "near enough for the bush", as they say.
