How to clean a fiberglass tub stained with hair dye?

It depends on the colour of dye used, some colours are more difficult to remove like red. Here's a few ways I found that you can try. I personally have my used any of them myself

1. Fill your fiberglass tub with hot water; add four to five tablets of denture cleaner to the water. Let the solution sit in the tub overnight. In the morning, drain and wipe clean.
2. Create a paste from borax and water. Use the paste to scrub the stain clean.
3. Dampen a clean cloth and pour a bit of baking soda onto it. Scrub the stain with the baking soda-soaked cloth. Rinse and wipe clean.
4. Spray the stain with 1/4 cup household bleach mixed with water in a 1-quart spray bottle. Allow the bleach to set on the stain until it lessens. Scrub clean with a soft cloth or sponge

Soft Scrub with bleach, Bar Keepers Friend

http://www.howtocleanstuff.net/how-to-remove-dye-stains-from-a-bathtub/

fill tub with hot water and lots of bleach let sit over night;easy off oven cleaner in blue bottle spray on let sit then scrub with hot water(fumes both ways use fans open doors windows);make paste of baking soda and pure lemon juice apply to stains let sit the longer the better;Comet cleanser scrub in on stains let sit the longer the better

Put bleach on it