Apply acetone to a white cloth. The white cloth will help you see the color transfer from the bathtub to the cloth.
Rub the cloth over the nail polish stain. Work until the stain is gone or until it no longer transfers from the bathtub.
Blot a small amount of paint thinner directly onto the fiberglass or porcelain surface if the dye from the pink nail polish is still visible. Use as little paint thinner as possible, but work until the stain is gone.
Mix 2 tablespoons of trisodium phosphate with 2 cups of water.
Immediately apply the solution to the bathtub to neutralize the paint thinner and prevent it from damaging the bathtub.
Rinse the area with clear water and let it air-dry.