Does anyone know how to change the color of a fiberglass bathtub???

This is a very difficult process. You must put a new jelcoat finish on the tub. This includes complete sanding and a very difficult process of spraying on a new top coat finish.
It is impossible to paint this surface as it will peel off with wear.
You will find that its much easier to change the tub or perhaps have a company that installs a new shell over the old tub do the work.