I'm selling rings to attach to your ballpoint pen so you can tell what color refill you've put in. The ballpoint pen in the photo is not included. Specifically for the Frixion ZONE. This is a best-fit ring created using a 3D printer, with numerous adjustments to the thickness, outer diameter, and inner diameter, as well as printing conditions. Simply remove the eraser cap, attach the ring, and replace the cap. The surface also has a bumpy texture to prevent slippage. I made these for myself, but I hope those who love the Frixion ZONE will use them. I'm offering them at a low price, so please give them a try. Colors: Red and Blue Quantity: 2 of each color, for a total of 4