Cartridges are ready to use, no mess, no fuss. Simply buy the right cartridge to fit your fountain pen, insert it into the pen and then you're good to go. However you don't get as much freedom when it comes to ink colour.  A converter will open up a world of colour options including glitter ink and flourescent inks. Simply fit a converter to your pen, then choose any ink you like in any colour or brand, they all work.  Then follow our guide on how to use a converter.