get down to your local Wilkinsons & buy "Grout Revive". Its made by Evo Stik. Its brilliant stuff & much, much, better than the grout pens that I really found quite hopeless. I used it on the grout in my bathroom & kitchen both of which were very grubby looking before. I've also used it on the grout on my stone kitchen floor tiles with good results too. Its around �3.15 for a tube. visit their site www.bostik.co.uk to know what you are looking for. I wouldn't suggest you use bleach