The grout you see when you're grouting tiles is usually not the grout you'll get. The most common tile grouts are made with Portland cement. Some are smooth and some contain sand. Smooth, unsanded grout is used to fill narrow seams, called grout lines, and sanded grout makes wider grout lines between floor tiles stronger. Both kinds of grout are mixed with water, which makes the grout appear darker. As the water evaporates, the grout returns to its natural color, which is the color of the dry powder before it was mixed.
Don't Trust the Color Swatch
Color swatches on the side of the grout package are printed with ink or applied with paint, so they are subject to inconsistencies. Sometimes, there is a powdery residue along the sealed edge of the cardboard container or paper bag. If there is enough residue present, use its color as a guide to make a better selection. If the swatch is charcoal and you want charcoal, but the residue is almost white, choose a darker shade. Chances are the grout in that container will dry almost white, like the residue. If you must use the color swatch to choose, keep in mind that there is no way to know whether the grout will dry lighter or darker than the swatch -- because the swatch is not grout at all.
Always Check the Batch Number
Buy enough grout for the whole job at the same time and check the batch number on the packages. Grout is subject to color variations from batch to batch. If you buy different batches and mix them separately, your grout will probably dry with a lighter color in at least one area, even if they look the same wet. If you can't find enough packages of the same batch, put all the powdered grouts together in an airtight container, and mix as much as you need a little at a time. Although the grout will still dry lighter, the finished job will be a uniform color.
Apply Enhancing Sealer
Apply enhancing sealer to help darken the color of dry grout. Although it may never be as dark as it was before it dried, the sealer will help deepen the color and block stains. The most common grout sealers will not darken grout because they are formulated to dry invisibly. Check the label to be sure yours is an enhancing sealer.
Consider Epoxy Grout
Skip the cement grout and avoid dramatic lightening with a product called epoxy grout. Epoxy grout is expensive compared to cement grout, but the durability and stain resistance are worth it for some homeowners. Epoxy grout is usually premixed and doesn't contain water. The first epoxy grouts made many homeowners and tile setters nervous, because it dried fast and stuck like glue to everything it touched. Some newer epoxy grouts contain detergents to make the cleanup easier, but they still have a shorter working time than cement grout. Once it is cured, it might be set for life, so clean up the residue as soon as possible.