Here is what Seachem had to say:
Thank so much for using our products, and also for your question regarding Purigen. Because Purigen readily removes color bodies from the water, it's quite possible that it picked up something that has actually caused the resin to become stained. Rest assured, however, that regenerating the Purigen per the instructions will fully restore its capacity to remove organics and color bodies further, even if it does not turn back to the pure white color that it was when new.