It is likely that acid from the postcard has affected the stamp paper. Alternatively, but very unlikely is "dirty water" paper. The former "China clay" paper variety, variously described in multiple ways including "gray paper" was found to contain no Kaolin clay but contained silica from water at the paper mill. It is now called "dirty water" paper. The examples I have seen are either grayish or have a white porcelain look. Because the definition was inconsistent, the variety has no particular identifying features except that the paper color may be whiter or grayer.