As others have written, shower curbs are generally made with some form of solid surface and most people use a piece of whatever solid surface was used for their counter. Therefore the issue of matching never arises since presumably the counter matches the other finishes in the room.
However I would match the floor tile in terms of it complementing the floor tile as most people would want as little a visual break in the flooring as possible. You can probably get a nice remnant inexpensively from any stone yard or your installer or designer. You could bring your floor tiles to see which looks the most visually seamless.
Q