[Austin W Hogan]

The PT gun 69017 discussed in another post in the hammer section is pretty original, and rebrowns itself when I use it at skeet or 5 stand on humid days. Wiping the barrel down with Hoppes in the cleaning process removes the yellow, and another rub with white mineral oil restores the color. I give all metal parts a rub with Rig as a final stage of cleaning. Rig is available again in a different looking can.
I had two shooters that were worn white that did show rust even when protected by Rig, on humid days or when rained on. I had both blued in the last ten years; the trigger gaurds are doing fine, but the barrel blue has come off the rib on one, and a few small pits have appearred on the other. It may be that modern bluing salts and pre WW II steel don't get along.
The big three frame lifter 12 that I shoot regularly was evenly yellow brown when I obtained it. The weekly rub down with Hoppes / MO / and Rig yeilded yellow patches for a year or so, but the PT pattern has re emerged. I contemplate dropping that big frame in the ultra sonic to see if the recovery occurs there.
Richard is probably right in his suggestion that some form of sulfur is the culprit in corrosion of newly blued guns. Find a GI ammo can or a big restaurant size mayonaise jar and clean it out. Put items from your shooting kit, paper towels you may use for wipe down or wiping your hands, and anything else you suspect in the can or jar and seal it for a week, then open and sniff. If it smells like farts or rotten eggs you have found the culprit. Newspaper can be especially bad.

Best, Austin