Personal preference - they both look great! But the black grout is more of a statement (which i love!)

What a beautiful tile! Rule of the tump - always match the grout to the tile. Specially with such a strong patterned tile, otherwise the grout also becomes a clashing pattern. I'd pick light grey to avoid pure white cleaning issues. I have these type of tiles in my patio, you will have to be seal them with penetrating sealer after installation, but other then that you will love them forever

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.

Doesn’t look like mould. If it had been used a bit more I’d say it was just body and shampoo/conditioner debris. Give it a clean and seal the grout with a sealer like enduro shield.