German 1947 Changling Or Something Else?

At first sight this stamp appears to be a Michel #40 in the BRD section.





But the colour is all wrong. The stamp is a light brown colour while Mi40 is a dark gray-blue or a dark greenish-blue. It appears to be postally used.

The back shows signs of thinning. There is also a round yellow patch, probably the result of the stamp being glued. Note: I remember as a child my mum getting mail from Germany where the stamps were glued although they had gum on them too.

Help anyone?

What is the "BRD" section?

With light or chemical damage, depending on the ink
I have examples of stamps going to dark brown
(on the maroon) side, from greenish blue (Japan)

Hi Rod. Thanks for the reply; it may be chemical damage although I secretly hope it is something more interesting. BRD is "Bundesrepublik Deutschland." I probably should have written West German (or even more precise; "American and British zone" section)