do you have any craftex rust remover? dab a little on a test area then dab a high pH solution ( ammonia) but i did some quarry tiles on a customer front step that had been badly stained with render although they washed it off is still left a bad stain. craftex rust remover was all i had on the van but it worked brilliantly ( although it was expensive)
any strong acid might do the same.
but I'm no tile expert, someone else might give better advice