To clarify, the shower walls will be the large white tile with gray veining to the ceiling and the rest of the bathroom will be the board and batten and blue paint .

My choice would be: White or lightest grey for walls and match the grey of the floor tile for the floor.

I agree with @Jennifer Svensson. I would not want to emphasize grout lines on those large wall tiles so I’d match the white of the tile as close as possible. Same for floor - I’d match the mid gray as close as possible. With the floor tile, you can see from the piece laid out what darker grout would look like - the shadows are dark.

Board and batten is used outdoors. What are you planning on doing for the bathroom?

Also agree to match the tiles closely

It’s a board and batten look, perhaps wainscoting would be a better description

Yep, wainscoting. :)