Remove stains on a fiberglass tub by pouring chlorine bleach directly onto the stains and scrubbing them with a sponge. Rinse the area with cold water, and scrub until the stains are lightened or removed.
Use a bathtub cleanser with some abrasive property, such as a cleansing powder. A liquid cleanser that has bleach in it can also work. Squirt or sprinkle the cleanser onto the stains, and scrub them vigorously. Rinse the tub clean with cold water.
Clean stains from a porcelain tub by making a cleaning paste: Shake some baking soda onto the dye stains, then follow up with white distilled vinegar. The two products react and create a fizzy cleaning product that naturally bleaches bathtubs. Scrub the stains, and then rinse the tub clean.