When replacing any tile, it's always best to use one left over from the original installation. Then you can be sure that the replacement tile will perfectly match the existing ones. If you don't have any extras squirreled away, bring the broken pieces to a well-stocked tile store, where you might be able to find a new one that's a close substitute.
Another option is to have tiles made to order. For example, North Prairie Tileworks, in Minneapolis, charges $30 a square foot (minimum order: 2 square feet) to match your broken shards, as long as the glaze color is in their collection. Custom glazes cost considerably more. Just make sure any replacement tiles meet the standards for ceramic floor tiles.
Or you could forget about trying to match the tile altogether, and fill the spot with an "accent" tile of a different color or texture. In that case, you might want to randomly replace a handful of tiles around the floor to make the fix blend in with the rest of the field.