Tile grout, the cementitious material pressed between individual tiles, holds them in place and prevents water from leaking through the tiles to the subfloor below. Tile grout comes in almost as many colors as tile does, so if you want dark grout, begin by choosing it based on the color it will be once it has dried. As time goes on, you can do a few things to make sure the color of your grout remains the color it was when it was new.
Types of Grout
There are two main types of grout, each with its own application method. In tile installations where the area between the tiles is less than 1/8 inch, unsanded grout is often the material of choice. Unsanded grout, as its name implies, has no sand, but instead is a mixture of Portland cement and a color. Sanded grout is used for tile joints of greater than 1/8 inch and is a thicker, more gritty substance. Because of the larger bits of sand in it, sanded grout takes on a bit of a cement-like appearance when it dries. With certain pigments, this make may the grout appear a lighter shade.
Most grout manufacturers include a small sample color of the grout on the side of the box or container. These colors are not always what they seem, so consider purchasing several small amounts of a few different kinds of grout and testing it with your tile. Seeing the grout dried and with your tile should give you a better idea of what to expect once the entire tile project is grouted, and help you choose a color that fits your needs.
If your grout has lightened over time, it may be caused by stains from hard water and soap residue. Use a commercial grout cleaner, sold at home improvement and hardware stores, to thoroughly clean the grout. Seal the grout, using a grout sealer that dries clear.
Sealing the Grout
All grout, sanded and unsanded, is porous. Porous materials that are exposed to water will, over time, stain and discolor. Many tile installers will seal the grout between tiles in kitchens and bathrooms to prevent future staining. If you regrouted your tiles or scrubbed them using an abrasive cleaner, you may have removed the grout sealant. When you reseal the grout, use a clear sealant rather than a colored version. If the grout has become very light, you can use a colored sealant to darken it, but make sure it's the right color by testing first. Sealant will help your grout remain the dark color you chose and stays clean and free from stains for years to come.