any stain remover should work ,I don’t know why a diluted bleach mixture would not work .After all ceramic colors are baked in after being glazed,but don’t take that chance.
try soaking in a Efferdent false teeth solution,over night .
Good luck
Google or Yahoo (cksq)
try a baking soda paste, let sit, wash/rinse well… make take a few tries. Good luck!