I think a classic dark grey grout for your lovely neo-traditional combination of tiles would make the most practical sense - white or light grey (or for that matter other colours of grout - say, yellow or blue) will get grubby on a floor, and, in my experience, black is (oddly enough) a bit of a nightmare to get clean as well (it also seems to get everywhere during the grouting process).

That said, visually, I think a bright flash of, for example, sky blue would look striking and unusual. But you’d need to be prepared to scrub regularly...

As an aside, have your floor tiles got a non-slip finish (or are they porous and not being sealed)? If so, take care when grouting to avoid getting them covered in grout and then having to go completely mad with grout remover after you’ve finished!

Theres a faux grout line in the tiles isn't there? I would try to match to that or use the same as the wall colour, it looks good.