I would define a "Changeling" as a broad term to refer to any change brought about typically by light or chemicals.
In the chemical realm, there are acids and bases, organic and inorganic compounds, etc. Some contain oxygen, some don't, etc. "Oxidation" reactions are only a subset of the reactions which could produce a changeling. Equating oxidation and changeling as the same is not scientifically valid.
I believe that the ink used for the orange stamp was not cleaned off properly before the run of olive stamps began.