Excellent Information on the Pilot Parallel Pen Parallel Pens

 We have this book in our Library       Parallel Pen Wizardry

Be aware that the ink that comes with the pens is fugitive - not light-fast.  Over time, it will fade.
You can reuse the empty cartridges and fill them with GOOD inks... or you can simply fill the whole pen barrel.

DO NOT use India Ink, and only use MOON PALACE Sumi (John Neal Bookseller) (Paper & Ink Arts)
Alice Young and Lisa Engelbrecht use FW Acrylic Black Ink directly in the barrel.
You can use J. Herbin inks for lovely colours!

Island Blue in Victoria carries the FW and Herbin inks as well as the Parallel Pens.

Alice Young provides Guidelines for each pen size on her web site.
She also says:  I generally use FW Acrylic black ink in my pens. Now, many people will tell you that you can't
use acrylics in a cartridge pen. And, indeed, if you fill the pen and put it aside for 6 mos. it might get dry and nasty.
But if you use it regularly, it will be no problem, and the FW has much more 'body' - not as watery as the Parallel Pen ink.

The pdf files are also here for your ease.
Red Pen     Orange Pen    Green Pen    Blue Pen
Each one of these pdf’s contains two pages. The first one is designed to be used for Foundational, Blackletter, Carolingian or Neuland,
the second page can be used for Italic, Fraktur and Roman Caps. Print at 100% for accurate sizing, though depending on your
printer margins, the edges might cut off. Scale to fit page and they’re just a bit smaller, but seem to work well, too.

also from Alice Young:  How to Refill a used Cartridge

Carol Dubosch shares her tips HERE.