Grout color decides whether people see the tile or the grid. Neither answer is wrong, but the choice should be made on purpose.
Match to quiet the wall
A grout close to the tile color lets the surface read as one material. It suits large-format and stone-look tile where the goal is a calm, continuous wall.
Contrast to show the pattern
A darker or lighter grout outlines every piece, which is what makes a herringbone or a small square mosaic worth doing. It also hides less, so the layout has to be right.
We bring physical grout samples to every backsplash consultation and set them against the actual tile in the kitchen's light before the color is final. Grout color is settled the same way on kitchen backsplashes and feature walls alike.