Penny,
If a tiled floor has been freshly grouted and a COLOURED grout has been used, it is important that the tiler keeps excess water away from the grouted lines as the floor is being cleaned off with, say a wet sponge or cloth. Grout will change colour and lighten up (bleaching) if the surface of the grout gets too much water on it,while the grout is still "green", ( not properly set). There is no way of getting the colour back to the original, now it has dried out....I'm not saying you did, but if you had cleaned the floor tiles with an over-wet mop/squeegee, before the grout has had time to set properly, this would cause grout dis-colouration too.
Roughcaster.