Pen company Pilot makes a multi-pen called the Coleto, usually only available in Japan but JetPens imports them. You buy a pen body and then whatever colors you want to fill it with, and it’s actually the multi version of the Hi-Tec-C (my favorite gel pen). I got the 3-color and 4-color bodies. The 4 is too large for the pen loop on my planner, but the 3 fits perfectly. I wanted it for my planner because I have appointments for three people to keep track of, and it’s easier to color-code the appointments. The Coleto is perfect for this. Also, it’s really cheap.
I don’t really want to use a paper planner, but it’s backup for my iPhone. My appointments keep disappearing from iCloud. Not cool. So paper backup it is…