So what was the explanation for the discolourisation? You could blame topps for the scratches, but only if the metallic mosaics were purchased from topps and if the grout was suggested/recommeded at point of sale.
I can't see how the grout drying off white and of a gray colour being there fault. Unless of course the grout has scratched particles of the "metallic" covering on the mosaics and these have coloured the grout in effect. If that is the case topps could take partial blame with what i mentioned above.
But to be completely honest, your tiler should of read the back of the bag first and i think he would of drawn the same conclusion, Not saying he's a bad tiler at all, but there is a warning of damage to tiles on the reverse of the micromax bags.