Much better than a cartridge
This was put into service in a Preppy pen. The choices were to continue to use a cartridge with the limited choices in ink colors, convert over to an eye dropper pen, or to use a converter. This converter allows me to use whatever ink the whim of the day might dictate. Easy to fill, and just as easy to empty when the need for another color ink pops up. Just clean out the section, and change the ink from neon purple to the darkest black. It only takes a few minutes.
A better option is that a full fill is not needed if only a few pages are to receive the new ink color. Just fill whatever amount you feel is needed for the task at hand.