Ah, Mr Barrell, you woke me up .
My only "proof" on the change from Eau-de-nil to Sky Blue was in an EMER covering Rolls B range engines. Without digging it out, I think the early B range units were Eau-de-nil then there was a change to Sky Blue around early 1950's. Guess this was a policy change on all engine. In an around 1974 I was involved in engine reconditioning in army workshops and most engines were Sky Blue when they came in, but I do recollect some very early Bedford 300 engines coming in for overhaul still in Eau. As the 300 came out about 1952, this will give some idea. I have seen new Daimler scout car engine parts boxed in 1954 and they were Sky Blue. What I would like to know is when did they start to use Eau-de-nil on engines?
Just a passing comment, you say Sky Blue was a rebuild colour, it was in fact the standard colour of engines from new in postwar A vehicles, Alvis, Daimler, etc.