I would use a light grey close in tone to the tile.

thank you. Any specific color grey? sometimes grey can look like cement

We can't judge the precise gray from a monitor -- you need to hold the samples next to your tile and see what appears to be the right tone and lightness. Also, there's not a right answer: some people like the contrast between pebbles/tile and white grout; others prefer to match the grout very closely to the tile, so that the lines disappear; some like a slight contrast either lighter or darker. Perhaps you should look at some photos here to see what style appeals.

It might be easier if all the grouts were to match, but you could also justify different approaches in the different contexts -- for example, maybe your matte shower tiles would look good with nearly invisible grout, while the "boards" and pebbles would like a little separation between grout and foreground...