Yes, the oranges in particular suffer from sulfurization. Oxidation is the wrong term as the ink pigment does not contain iron. The orange color turns brown from smog, cigarette smoke and other airborne pollutants. (I don't know if excess flatulence has an affect on the color.)

You can recover the true color of the stamp by giving it a bath in water peroxide (3% or 5% solution). Maybe a half hour, an hour or so should do. After the color returns, wash it off in cool water.

 
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