Lay your fabric shower curtain on a flat work surface. Carefully blot the fabric with a paper towel to absorb excess hair dye. Discard the soiled paper towel.
Cover the remaining hair dye stain with heavy-duty laundry detergent. Allow the detergent to soak into the fabric for 10 minutes. Rinse the fabric shower curtain while rubbing the detergent into the fabric.
Mix 1 cup of ammonia with 1 gallon of cool water in the first bucket. Hold the hair dye-stained area of the fabric shower curtain taut over the top of the second bucket.
Pour the diluted ammonia solution over the remaining hair dye stain and into the bucket. Rinse the fabric with cool water to remove the hair dye and ammonia solution.
Launder your fabric shower curtain in warm water. Add heavy-duty laundry detergent and all-fabric bleach to the wash cycle to remove any last trace of the hair dye stain.
Look at the shower curtain to ensure the stain has been removed before drying it. If any hair dye stain remains, soak the curtain in a solution made from 1 gallon of cool water and 1/2 cup of all-fabric bleach. Launder it again and dry it according to the directions on the care label.