Many, if not most older lubes can and will turn to "varnish"
This includes CLP Breakfree.
Years ago, a National Guard tanker gave me a sample of CLP that had just been issued for use in firearms, including the cleaning of their tank's main 105mm gun barrel.
I used this early CLP to preserve a Colt Detective Special for storage.
A year late, the CLP had "varnished" into a sticky, gummy layer in tightly fitted areas.
Varnished lubes invariably have a yellowish color and appear as an obvious sticky-to-dried layer, which is easily removed with solvent.
Again, I have two 1980's Colt stainless Pythons, both with the stain in the crane area.
Again, I've seen this discoloration on many Stainless Colt revolvers of this era with the stain typically appearing around the crane area of the frame, the underside of the ejector, and the rear face of the cylinder in the ejector recess.
In appearance, this stain runs from a mottled thin gray-black to black color. It's as smooth as the surrounding metal, with NO roughness or "crusty" texture.
It appears as "in the metal" stain, NOT corrosion.
For some reason, this discoloration is prevalent on Colt stainless revolvers of the 1980 period.
I have rarely seen it on other makes of stainless firearms, and, of course, never on a blued firearm.
My experience is, this discoloration is a Colt stainless revolver phenomenon, of the 1980s period.
Whatever it is, it ISN'T corrosion or lube varnish. It is removable ONLY by polishing the metal.