I tiled my kitchen countertops and backsplashes. The countertop tile is great because it is porcelain - so no discoloration from the grout (I used brown). The backsplash is tumbled marble and it is darkened from the grout. Is there any product I can use to lighten up the tile? I am afraid to try anything for fear of ruining the marble. I should mention that I put three coats of sealer on the tile before I grouted.