I recently revitalised a bathroom with a bottle of white Universal Sealant grout colourant and a tube of Silicon. Tiles were 600x400 over a 20 sq m wall area and it took me four hours to clean and re-vitalise. Everyone who sees the bathroom think its been recently re-tiled. However, I can frankly see the limitations (it passes the 18" test that my brother-in-law uses, but closer in you can see its painted) but if you are selling a house and want a quick refresh its perfect. Otherwise I fully endorse the comments that re-tiling is ultimately better.