It is possible to paint the inside of a toilet. First, it is necessary to prep the toilet bowl by emptying it of water, cleaning it, and sanding it with sandpaper.
Use a trisodium phosphate cleaner to scrub the toilet to remove any debris and dirt, then rinse with water and allow the toilet to dry. Next, sand the toilet bowl with 120-grit sandpaper. Once the preparation is complete, use acrylic latex spray primer to coat the toilet bowl and toilet, and allow the primer to dry. Finish up by applying epoxy spray paint to the bowl, and allow it to dry. Use caution when handling all chemicals.