Sorting through my Thai stamps I noticed this colour change in the 1961 Thai 2 baht definitive. Look at the two stamps on the right. Red is correct while brown s not.



I've seen similar with Thai stamps from the 1980s. In that case it was certainly due to a long exposure to sunlight. (UV light I guess.)

I know the latter was sunlight because I had left a used envelope sticking out from a pile or paperwork for a very long time where it was exposed.