Jerry, a lot depends on what type of shell you have; is this a fiberglass or other man-made material? Or a built on site concrete & tile?
If it's a man-made, you may be able to perk up an oxidized finish with some polishing. If you're primarily wanting to change colors, this can be done with slick finish fiberglass shells, but some of the other man mades might be a problem.
What's the age of the tub? You might find it would make more sense to look at trading in for a new tub.