Hi Turkish mate.....That Lithofin product will be spot on for them...and will intensify the colours as well....
I would seal before grouting as the micro flex might stick to some of the open pores thus leaving that white speckled appearance on them...
If you want to seal afterwards then try on an inconspicuous area and see what result it leaves with the micro flex.....
Here's the manufactures instructions for you.........
A ready-to-use special product from the LITHOFIN-Range for ceramic, natural-and engineered stone. Lithofin Stainstop PLUS contains high-quality water and oil repellent agents based on siloxanes and dissolved in solvents. Special additives permanently intensify the colour structure.
Lithofin Stainstop PLUS sinks into the stone and reduces the capillary absorbency so water, oil and grease can no longer penetrate the surface. These materials remain on the surface and can be wiped off before they cause stains. In addition to protecting the stone, Lithofin Stainstop PLUS darkens it thereby invigorating the colour structure.
Field of Use:
For making absorbent ceramic, terracotta, natural-and engineered stone surfaces stain resistant while at the same time intensifying their colour structure. Highly suitable for weak to medium absorbent natural stones like granite, gneiss, quartzite, slate and limestone with rough to honed surfaces. Conditionally suitable for polished stones.