Navy and Marine planes also tended to be around salt water so painting protected the stuff like rivets and unprotected metal from corrosion.
it still happens, horribly. Nothing is more corrosive an environment for aircraft grade aluminum, magnesium, and copper alloys than being on an ocean-going ship taking salt spray from uncounted numbers of waves. Today's modern A/C use corrosion-resistant alloys, corrosion inhibiting primer coats, corrosion preventative topcoats (2 primer, 3 top) - and still suffer from large to extreme cases of dissimilar and intergranular corrosion (the worst two of the bunch) - along with all other types as well, which are not as debilitating to the strength of the materials.
USN/USMC went with the dark blue topside, light gull grey lower surfaces, as a method of camouflage - at the beginning of naval aviation, most A/C were doped and stretched fabric over aluminum alloy inner structure - this really did not change until the F2A Buffalo, the SDB Dauntless and the SB2C Vindicator were finalized as onboard standard types. Even then,many later war a/c still used doped fabric for a main skin, rather than stressed metals, including the TBM Avenger, and the F4F-3 Wildcat models. skin fabric dope dries to a light cream color in most cases (this has changed in the last 40 years or so as newer dope material components have bene used to assist with matching paint schemes.). Light beige or cream, solid anodized aluminum, or mottled greens, were simply too easy to spot from above against an ocean backdrop.
Brad M-V points it out above, but most "silver" skins were anodized aluminum alloys, and were far from shiny silver in their appearance up close. It was the most efficient way to both save weight and to protect the a/c from corrosion. Side benefits were a significant cost savings per unit as no paint was expended.
