I have never heard of painting a bathtub, but I do work in the yacht repair business, and we paint fiberglass boats evey day. Therefore it seems to me that it shouldn't be aproblem at all. You have two options that I think should work well, the first is using gel coat. This is what was originally done I'll bet. it is just pigmented polyester resin.(Fiberglass is polyester resin over glass fibers). if you want to do this method youwill need to either spray it on, or get good at sanding and polishing. The other way to recolor fiberglass would be to use a linear polyurethane, or LPU as it is commonly called. This is what is generally used for painting fibergalass yachts, but it to should be sprayed on if you want to achieve a good smooth finish. for LPU it wil cost you anywhere from $100-$200 per gallon. Which is more than enough to paint your tub many times. Gel coat will cost you around $30.00/gallon if you want white, otherwise it will cost you for pigment as well.