These are somewhat common to find. Two possible explanations. First is the medicine in the package reacted with the stamp and caused the change in paper color. Second is that it was done purposely by a philatelist. From the 1860's into the early 1900's there was a collector named J.N.T. Levick who was reportedly a chemist.
He did a number of strange things to stamps to change their appearance such as chemically changing green to blue, blue to purple, etc. He also experimented with paper colors on the taxpaids. It is not uncommon to find his name stamped on the back of taxpaids.
It is possible that he may have played with this stamp or it could have been somebody else experimenting.