Grout can be painted with a special product called grout stain to darken and change the colour of the grout. The special paint is absorbed by the grout so that it will not flake off over time in the same way that regular paints would. If you're unhappy with the colour of your tile grout but do not want to deal with the hassle of a brand new tile installation, grout stain can give your tiled surfaces a face lift in as little as one afternoon.
Mix 3/4 cup baking soda and 1/4 cup water together to form a paste. Apply this paste to the tile grout with a toothbrush and let it sit for a few minutes. Scrub the paste into the grout and rinse the paste with clean water. Wipe the tiles and grout with a clean towel to dry the surface.
Paint grout stain directly onto the grout using a paintbrush slightly smaller than the width of the grout. Work slowly and carefully and wipe any mistakes or spills off of tiles immediately with a damp rag.
Let the grout stain dry completely. If the colour is still too light, you can apply a second coat of the grout stain in the same manner.
Glazed tiles make staining grout much easier because mistakes will easily wipe away from the face of the tile. Unglazed tiles may absorb the grout stain and become discoloured so be extremely careful if you are working with unglazed tile.