TLDR; If you are getting a pochi with a white body and don’t have a good white marker pen that works well with urethane, then yes. Otherwise, no.
If you are going to get a pochi with a black body, you wouldn’t need a Mitsubishi Paint Marker. On the other hand, if you’re getting a pochi with a white body, you might want to get one.
pochi sandwiches three sheets of PORON foam, and the sides of the foam will be visible from the outside. PORON is available only in black, so you want to color them in white to make them look nice. Otherwise, you will have a zebra pochi.
Now, not any white marker pens work for this task because what you’re coloring is not just a sheet of paper. You need a marker for urethane foam which bends and compresses on your key presses.
It doesn’t have to be Mitsubishi Paint Marker, but you should avoid water-based or dye-ink markers. You want oil-based pigment-ink markers. We have tested different white markers available in Japan and found this one the best.
If you are curious about the difference between oil-based vs water-based, and pigment-ink vs dye-ink, take a look at this explanation by uni : What is the difference between oil and water, dye and pigment .
TLDR; You don’t have to, but if you don’t have any good marker pens around that you can color your pochi’s PCB and stacks, why not?
It’s just for coloring the sides of the PCB and the stacks. uni POSCAs are one of the most widely accepted marker pens in the Japanese keyboard community for this purpose. We like it because it’s got a wide variety of colors, and the colors it produces are nice and vivid.