well the shell disine dosent come off. and i wound try paing remover if you alredy painted it because it may get into your tama and break it! so your going to have to paint over it if you want to paint.
The only way to take the paint off would be to put it over comething VERY hot, like a fire. Although, that would break the tama, too. I would suggest just painting it whatever color you want to be the background color, then make your own design.
If you mean just the pattern on the shell (not the base colour) then you can do it using an eraser and erasing the design, pressing firmly. Then you can paint over the base colour using whatever you want.