Cement tile is very porous. It is crucial that before you sell cement tile, you carefully read the installation instructions from the manufacturer and ONLY sell the recommended sealer and grout. If something goes wrong during installation, the manufacturer will not assist if their instructions were not followed.
The image above is an example of cement tile that is supposed to look like the image below. Even though the installer pre-sealed (which is correct), unfortunately, they used the incorrect sealer and grout. This tile will either need to be sanded down and re-sealed or ripped out if the grout pigment has stained too far down into the cement color layer.