Thanks Steve. We selected the colour from actual grout samples (not a colour chart) and yes we also had the tile with us at the time... the lighting there was pretty poor though :/ A little confused, you say that raking out joints is no big deal but then that using grout sealear is very tedious... do you mean removing old grout not so hard but applying new grout is harder? Now I'm starting to wonder if they did indeed use the wrong colour, as we had also selected another even lighter grey for the wall tiles and wondering if they used that on the floors too... and even if they did, I'm not sure what can be done about it now :/

It's a lovely tile! I'd go light-mid grey (basically like the picture!), if you go dark then it's pointless having the interesting hexagonal shape and the veins will look a bit odd stopping and starting, and if you go too light it will compete with the veins in the marble and look to busy.

Thanks for that, rach19. I a man no expert but it looks like both issues are happening. I think I detect efflorescence and the grout itself possibly blended poorly. Please call out whoever built your pool (not the tiler as he's not the builder) and complain and say it's not acceptable. There's no way this should be happening so soon.

Sounds like a major hassle to me! White grout paint is usually for cleaning up grout stained by mildew, etc, not to go over a completely different colour. I am really surprised that your tilers did not consult with you about the grout colour before they went ahead. If you are really unhappy, and they acted without permission, you might be able to get them to redo. Otherwise, you may have to live with it and eventually wont notice it once the space is in regular use.