Purple Stamps Rubbing Off Color

My question is about purple stamps such as the beautiful
Eleanor Roosevelt USA stamp. These stamps leave a purple powder on paper. They seem to leave a purple powder that shakes off or comes off easily.Why does this happen? How should they be handled and mounted?

This is common enough for many engraved stamps of relatively modern times, probably due to the ink composition or inking, sometimes due to bad storage. Dark blue engraved stamps can do the same thing like the later 20c Eleanor Roosevelt stamp.

Obviously, if you have two-sided album pages, you don't want color transfer and should either keep Eleanor separate or use glassine or acetate interleavings if you don't already. A stamp mount will also prevent color transfer.