If you already had them 4 tiles or from the same batch purchased that were used It could be a drying problem and might blend in overtime. If you fitted underfloor heating this might help once adhesive has had chance to cure. Im at a loss that your marble was not sealed prior to grouting and as mentioned fitted with epoxy grout. If all else fails you could seal the tiles with a slightly colour enhanced marble impregnation to help blend what you have. also what wetroom tray was have you used? just dont recognise it.
Discoloured Marble, New Install. Help Please !!
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