Because who doesn't love taking apart a pen and re-filling it with a different color of ink?
+ a used-up Pilot Precise rollerball pen to dismantle
+ pliers, probably needle-nose
+ a small cup of water (like a rinsed-out yogurt container would be fine)
+ a big fat rubber band (or one of those rubber grippy things you use to open jars)
+ a syringe (or a pipette, or an eyedropper, or maybe even a plastic drinking straw could work, but it'll be messy!)
Wrap the rubber band around those disc-things right behind the white plastic piece, and grip the rubber-band-wrapped-plastic with the needle nose pliers.
Pull. Hard. I found it was easier to grip the pliers with my dominant hand, and hold that steady while trying to yank the body of the pen off, sort of towards the side, so that nothing goes flying off into the distance, and also you won't accidentally punch yourself in the face.
You can see that I used enough force to leave a mark on the plastic. It's not quite as shiny anymore. If I figure out how to do this without squeezing the bejeezus out of it, I'll let you know.
Next step: rinse. Leave the feed in a cup of water to soak, and rinse the body of the pen out in the sink. Shaking the water out of it helps - just point the open end towards the sink. You're done when the water runs clear. The water soaking the feed might need to be changed a few times.
The fun part: refill with ink! It can definitely hold 1.2 mL (yes, I measured!) and maybe a smidge more. The syringe with a blunt needle is the neatest way to do this, followed by a pipette or an eyedropper. You could try to pour your choice of ink (hahahaha, no, don't!)...or maybe employ the straw trick. (Stick straw in liquid, cover top with thumb, lift out of liquid, position over pen body, release thumb....and probably over-fill by a lot. Prepare for spillage if using this method.)
A note on ink: fountain pen ink works really well for this. I'd think of any other kind of ink as an experiment. My guess is that acrylic inks would clog it up eventually, but hey, you were going to toss this disposable pen anyway, right?
My black pen now writes purple. What color will you use in yours?