It was a protective 'primer' type of paint VB. Most countries used (and still do) a zinc, or zinc chromate based primer to prevent oxidation of the aluminium and other alloys, and provide a hard wearing finish, resistant to scratching etc. Depending on the mix, this was invariably a green, yellowish green, or a sickly yellow-with-a-hint of green, due to the ingredients. The British 'Cockpit Green' was a paler, greyish-green colour, but still in the same 'family', again due to the 'ingredients' of the metal powders in the paints. German colours were more towards the grey initially, but with a green tinge, whilst Japanese factories tended to use a 'metallic' paint, a little like a metal flake as we would know today. Although not the same process, the aim was similar to galvanizing.
The modern primers used today utilise a similar base, although, due to the inclusion of more synthetic materials, they tend to be a paler, yellowish grey, whilst cockpits etc are finished in synthetic, very tough coatings, many of which tend to be grey, or a bluish grey....and rather expensive, compared to 'traditional', everyday household paints.