Tile Grout Wicking

We just had new tile and plater done. We chose a light tan grout to go with the 6x6 travertine look tiles. When they filled the pool, the grout immediately turned very dark from the water line up to the first horizontal grout line. Then the darkness migrated along the horizontal line. Then it started to travel up the next vertical lines, sometimes halfway up, sometimes all the way up. It looks bad. Our pool man doesn't know why. He has offered to saw cut the grout out and replace it. My questions are: Does anyone know what the problem is? How do we prevent it from happening again.
Thank you.