Removal of excess grout and film in the cleaning stage will be facilitiated by the use of a grout release coating to the surface of the tiles. This is recommended for all unglazed tiles. Job conditions must be controlled. When grout cures at different rates, shade variations will result. Cooler temperatures make grout dry slower and darker in color.
Warmer temperatures cause faster curing and subsequently lighter joints.
Try to maintain uniform temperature and ventilation. Never sweep sand or excess mortar into the joints. This causes a non-uniform joint depth, increasing the possibility of cracking and discoloring. How to properly mix grout. The amount of water used to mix the grout is of utmost importance.