Acid rinse and dry as quickly as possible. We have had it, though not such a gap between cleaning & finding out, and it fully reversed. one was an undyed linen, saw it happening on the bits the blowers weren't getting to just as I was finishing. the only acid I had on board (in the porty days when we didn't routinely acid rinse) was rust remover - just sploshed a fair bit (no idea about diln rate or acid strength) into the solution tank and re- cleaned, no detergent. The other one I can remember was orange cotton that we got called back to next day, just rinsed with standard TM add rinse.