Why does grout go dark when the floor is mopped every night?
Because the mop is part of the problem. Grout is porous and sits slightly below the tile, so dirty mop water runs into the lines and dries there. Traffic grinds more dirt in, and over months the lines go grey even though the tiles look clean.
A regular mop will not lift it. Specialist tile cleaning scrubs the grout back so the lines match the tile again.