Walls – How to make light blue paint more interesting?

The two standard ways to strip paint is heat or chemicals. Both are labor intensive and messy, and chemicals are caustic. I assume you are planning to strip the paint only because you are left with an uneven surface after it was already partly removed? Otherwise, just paint over it right?

It might be easier to lightly scrape to remove loose paint, then skim coat the whole surface with finishing plaster. Some light sanding afterwards, and you're ready for prime coat.

If the tub material is fiberglass (and most commonly are) the best way to remove any blemish is with an orbital sander and some 300 600 and 1000 grit Wet and Dry sandpaper. Using a spray bottle filled with water wet the stained surface. Starting with the lowest grit sand and keep the area wet. Let the sander do the work. No need for hand pressure. Watch how fast the stain is being removed (it should go quickly. Move through the grits and spray to keep surface continuously wet. The paper is of such a fine grit it will not leave any discernible scratches. When completed apply a cleaner/sealer for fiberglass.