If you have a small, narrow crack in a tile, you may be able to fill the crack with paintable caulk and, if necessary, touch up the caulk with an artist's paintbrush and acrylic paint.Try this first.

If the repair is too obvious, you'll have to move on to Plan B, which can be quite a bit more involved. Plan B is to remove the tile and replace it with another just like it. If you happen to have matching tile around, you can do this. Otherwise, you may have a very difficult time finding a replacement tile that is a perfect match. See Replacing a Cracked or Broken Tile.