Pete,
Partly right, but the first pastel shade used, was Eau-de-Nil, which is a very pale green. Around about 1950, this was changed to another BS standard colour called Sky Blue. The term "duck egg blue" is incorrect and often used. Quite why the engine bay of this vehicle been painted one of these colours, I do not know, but it is possibly a Royal Navy requirement, as RAF vehicles used to have cab interiors painted Eau-de-Nil.
Your mention of reasons for painting engines these colours, I believe that fluid leaks were one of the main reasons, and anti freeze / coolant weeps are clearly visible.
regards, Richard