That's one stamp I do not have in my collection
generally speaking, any problems met with regarding colour IN A USED STAMP, is problematic, it can be damaged by all sorts of conditions.
It mostly depends on how the stamp was removed from the cover and stored.
Yours appears to have been damaged by either light exposure or chemicals.
rending the stamp damaged.
here is one from a similar set. (with Faux Australia Post authorised Postmark )