The reason: dirt that imperceptibly collects in tile seams over time. No matter how religiously you seal the grout and no matter how many old toothbrushes you use up, you are bound to end up with dingy, discolored, and disgusting grout.
The ultimate solution is to entirely remove the grout and lay it again. And while this is less difficult than one might imagine, it still does involve power tools, manual scraping, and re-grouting. One way to avoid the mess is to preserve the grout but change the color. You will particularly want to do this if the grout is in good health--stable, solid, and sealing. Why rip out functionally good grout only because of discoloration?
Grout Colorants 101
Grout, when it is colored, is usually colored before it is installed. Small amounts of color dye are added to the wet grout and mixed in. In addition, some grout comes factory-colored, though the range of colors is limited.
However, grout can also be colored with dyes after it has been installed. Grout's deficiency--its porous nature--becomes a plus when you apply a grout colorant because this allows the color to quickly soak in and for the color to stay fast for a long time. Not only that, grout tends to become more porous as time goes by.
To distinguish them from colorants that are added to wet grout, these retroactive grout colorants go by various names such as grout refresher, grout renewer, and grout reviver.
Brands: Grout Colorants and Renewers
Some of the more prominent makers of grout colorant-renewers include:
Artist's Hand Not Required
The greater the difference between the grout's porosity and the tile's resistance to grout colorant, the more successful this project will be. When applying grout colorant, you will be using either a special grout brush or a clean toothbrush. A steady hand is required to keep the colorant fairly close to seams, but slop is inevitable.
Products such as Aquamix Grout Colorant, if they lap over onto the tile surface, can be scrubbed off of the tile face two hours later with a nylon-faced sponge--provided the tile is well-glazed.
Tile pros uniformly recommend gridding off a work area, coloring that section, and then stopping for the morning or even for the whole day. Next time, grid off another section and work on that. Impatience leads to irreversible errors. You can grid off the work area with blue painter's tape, as it is easily removable and will not leave a tacky adhesive residue.
Not All Grout Can Be Colored
For the most part, you will find sanded or unsanded grout lines in your tile. But not always.
The grout needs to be receptive to color. Sometimes you will find natural stone countertops where the tiles were "grouted" with silicone or other inorganic materials that will not be receptive to the colorant. Any type of seam that easily sheds water will not be appropriate for grout coloring.
Why You Should Re-Color To a Darker Color "Resembling Dirt"
What color should you choose? Generally, darker colors fare better than lighter colors. Retired tile contractor John Bridge sums it up nicely: "Invariably, there are people who do not heed my advice about choosing a grout color resembling dirt when they have their floors tiled. Floor grout gets dirty..."
Regional differences can affect your choice of grout colorant. For example, in Seattle, Aquamix's Buff or even Linen White would most closely approximate the light-colored soils of those marshy areas. However, in red clay regions of the Southern U.S., Wheat, Smoke, or Cinnamon may be more appropriate.
Extending Bridge's advice, one could say, "a grout color resembling dirt or darker" to cover all bases.
Be careful, though, of setting up a color contrast that is too dramatic. For example, black grout against white tile would create a grid-like appearance that some homeowners may find unpleasant.