His change in color is most likely directly related to the treatment. Any time you treat the tank with meds, it will affect your fish somehow. Test your water daily for a few days or until he starts looking normal again. Also, go over the instructions again to make sure you did not overdose with the meds. If you did, just do a 35% water change every other day for a week.
He might be sick try feeding him a different brand and check if the water is safe.