When you've cleaned your grouting and it still looks discoloured get some water based white emulsion and water it down (60% paint 40% water) then use it to wipe over the grouting. Simply wipe off the excess from the tiles and the pigment in the paint has repainted your grouting. White!!! The same can be done with floor tile grouting by using the appropriate colour of emulsion. I'll never scrub or use those grouting pens again.