Our pen uses D1 ink refills. It can be found in your local stationery and book stores. The refill dimension is 67mm x 2.3mm.
The ink colour available right now is black. More ink colours (blue, red, etc) of your choice will be revealed when the second reward tier is unlocked.
The size is 1/8 inches (3.175mm) for both Phillips head and flat head.
Don't see the answer to your question? Ask the project creator directly.