Pour 1/4-cup of whiting into a bowl. Add 2 tablespoons of ammonia to the powder and stir the ingredients to make a paste. Add more ammonia, as needed, to create a thick poultice.
Smooth the poultice over the hair dye stain on your porcelain sink. Leave the poultice on the porcelain surface for one hour.
Remove the dried poultice with a paper towel and discard.
Scrub the porcelain surface with a nylon mesh sponge. Rinse the surface with water to remove all poultice residue.
Treat any remaining hair dye stain on the porcelain with bleach. Moisten a rag with bleach and lay it over the stain for five minutes. Remove the rag and rinse the porcelain with water. Repeat this step until the hair dye stain is gone.
Dry the porcelain sink surface with another rag.