So, a quick experiment. 4 Scott 907's:
First - original as removed from the envelop
Second - soaked for 10 minutes in a 3% solution of H2O2
Third - soaked in very hot water for 10 minutes, ink bleed is apparent
Forth - stamp with color in question
It doesn't appear that soaking the stamp caused the color to change or at least not dramatically. I'm of the opinion that the stamp was printed in a Lake color.