Yellowing tile grout is not all that uncommon in most households. Having to deal with grout that is more “brown” than “white” seems to be part of the nature of older grout materials.
The first thing that I would do is to determine whether the discoloration is on the surface of the grout. If it is a surface problem, then bleach should brighten it somewhat. Ordinary household bleach is the simplest thing to use, and you probably already have some in the house.
Some of the grout whiteners are simply bleach solutions with detergent additives; this is why they don’t have a list of ingredients on the label. Others are more like paint and add a layer of white on top of the grout. One problem with using this type in older houses is that the grout whitener is pure, bright white. The tiles, which appeared to be pure white in comparison to the dingy grout, are revealed to be more cream-colored when the grout is painted bright white. These products are pretty permanent, however.
To see if the stain is on the surface or whether it has gone through the grout, pick away some of the grout in an out-of-the-way area. Dental tools are great for digging out the grout. You can purchase inexpensive dental tools at flea markets and drug stores. I like the one that has a slanted end with a hook at the tip. These tools are made of high carbon steel and are incredibly strong for their size.
If the yellowing has gone deeply into the grout it may be necessary to remove the old material and re-grout the entire tile area. Believe it or not, grout is not all that hard to remove using these hooked dental tools. It’s just a time-consuming, somewhat tedious, boring and laborious task.
Carefully remove all the old grout. Be careful that you don’t dig down into the wall surface. Make sure all the joints are clean and free of any loose pieces of grout. Now you’re ready to regrout the tile.
Grout mix can be purchased at most home improvement and hardware stores. It comes in various size packages, and the coverage area is usually indicated on the package. Measure your wall area so that you know the square footage before you go to the store. In addition to the grout, you’ll need one or two heavy-duty sponges, also available where you purchase the grout. You may need to buy a 4-inch wide putty knife or drywall knife. I use the knife to scoop out the grout mixture onto the sponge.
Mix the grout compound according to the label directions. Wet an area of the tile, making sure that the grout area is moistened. Use the knife to lift out a small amount of grout mortar (3 to 4 tablespoons) and spread it across a moist sponge.
Using the sponge, work the grout into the joints in a back and forth motion. When the first daub of grout is worked into the joints, replenish the sponge and work on another area.
This operation goes very quickly. The sponge makes the grout fill the void between the tiles and leaves only a wisp of grout mixture on the surface of the tiles. Once the grout begins to dry (set up), use a clean moist sponge and wipe away the film of grout from the tiles.
Let the grout dry for about 24 hours. The joints can be sealed using lacquer and a small brush like a watercolor brush. Clear fingernail polish works well, too, and has a built in brush. This job is tedious at best, but it can prolong the whiteness and reduce the chance of mildew forming on the grout.