Garden Tiles- any ideas what might have caused this?

Hi,

These were laid by contractor c6 months ago, now discolored on edges. Any idea what might have caused it (close up best to see)? Caught in middle, contractor blames tiles (and supplier), of course supplier says contractor didn't know what he was doing. So who pays to relay?

Is it the tiles themselves? Is it the grouting? Could contractor have used grout for internal use outdoors? Any help appreciated on what may have caused the discoloration. Thanks.