A. Mike Ducato: It is of the utmost importance to seal newly installed grout to prevent discoloration over time. Excess water used in the installation can cause this as well. The best way to insure against discoloration is to make sure you’re using grout that has not exceeded it’s shelf life and follow manufacturers suggestions for recommended additives while preparing to install your new grout. Finally, should you have discoloration, mix two cups of bleach to a bucket of water and apply with a brush to clean and restore your grout. Afterwords, apply a good sealer. Remember that oftentimes the more the sealer costs, the more probable it will do the best job. Your local tile store should be able to recommend a quality sealer and one that’s not too difficult to install. The last step is to fill the corners with silicone based colored caulk to match the tile for a water tight job.
Read more about grout discoloring