The choice is determined by the overall colour scheme of the kitchen in the case or white tiles or the colour of the tiles.
White tiles with gray lines.
You want to go with the flow in grouting.
Light grey has a similar effect to softer whites it defines the tile and gives subtle contrast.
Vapour grey and bone grout colours both fit the bill.
I can google of course just love using inspiration off your page more since i know it will be better curated than a google search.
White grout is a risk in bathrooms though as everyone mentions.
Ok if you have the white tiles in my opinion you should go with the white.
Use a non sanded grout to protect both the marble and the grout from discoloring too much.
One is white with a hint of grey and the other with a hint of beige.
Would have loved to see more pictures of white tile with off white grout or varying shades of gray grout for those of us who don t want pure white but don t like the black look either.
The darker the grey you select the more contrast you will get and the more the subway grout line will become the attention grabber rather than the tile.
It should usually just compliment and fall into the background.
When white tile is paired with white grout the lines between the tile and grout will disappear.
The entire surface will blend together to give a uniform aesthetic.
B c the gray would then do what i explained the white would do in the last one.