The best you might do is to find a good bodyman who works with acrylic finishes and fibreglass, but it is going to be very difficult to repair spots and get them to truly blend in with the overall finish. If you want a really nice job, you will likely end up either refinishing or replacing the entire enclosure.
Call a bathtub refinisher that fixes chips.
He will be able to get or tint the color you need.
I could tell you how to do it but it would be cheaper to call someone.