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

My suggestion is first talk to the builder to see what instructions he gave the tiler as i'm sure he doesn't want a dissatisfied client. Most tilers will check with the builder before grouting especially when a colour is not specified in a contract. I would then ask if the grout could be replaced, from the looks of it it will only be a couple of hours work. There is nothing worse than having to look at something you are not happy with. I would not use the paint product as it will alway look like it is covering a mistake. White will look better. Make sure the mixer body is waterproofed or you will have a leak in your wall, the membrane should lap onto the body from the wall if it is done well. All the best.

Thanks Nicole!! We actually went for white grout too in the end, and stacked them (mid century modern house, we thought it fitted better overall). I agreed with you that I wanted the detail of the tile to shine, not the shape of the tile. We still need to paint the rest of the room, etc., but I'm pleased. We had them sealed and used a supposedly more stain resistant grout (Davco Colour Elite). We loved the designs so much we put the 150mm blue jeans version in our pantry!! Thanks again for your reply. Enjoy your new space.

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.