Difficult to tell without seeing them in the flesh, but they look to be either ceramic or porcelain. Good strong alkali cleaner, bit of dwell time, good scrub, rinse off thoroughly, et voila. Get a grout brush on the grout while your at it.....but, if the grout looks like it's been replaced over the old grout, then check it doesn't just scrub off - if it was done badly it may have not keyed to the old grout.
Chris