Mixing bulk inkjet printer inks produces pretty colors:
The small spots show the colors on paper (with the vials in a different order):
Three of those vials contain the original CMY inks, taken from a trio of small generic inkjet refill bottles.
Mixing 1:1 ratios of two inks produces the expected red / blue / green primaries.
Six other colors came from 2:1 blends of two inks and, except maybe for that purple over on the right of the top picture, aren’t worth the aggravation; plotter drawings don’t score higher for having a rich color palette.
In principle, I could dilute the mixes with water (alcohol? vodka?) to produce less saturated colors, but for plotter ink absolutely nothing exceeds like excess.
The CMY and 1:1 (= 0.5 ml each) vials should contain 1.0 ml and the 2:1 vials hold 0.9 ml (= 0.3 + 0.6 ml), but I didn’t sweat the small stuff and there was some, ah, spillage along the way.
The vials are 1.5 ml perfume sample vials from the usual eBay supplier: 50 of the things (with 10 squeezy plastic 3 ml pipettes) set me back nine bucks delivered. Refilling a plotter pen requires maybe 0.05 ml, so each vial holds 20-ish refills with plenty of headroom.
Uncapping and recapping the vials inside a towel makes a lot of sense; the ink makes its way between the cap and vial, creeps up to the lip, and spatters as the lid snaps closed. Fortunately, that t-shirt was getting on toward worn out…
Memo to Self: Do not fiddle with magenta ink immediately before chopping the supper vegetables.
The Smell of Molten Projects in the Morning 
Oh, I can imagine the rainbow colored mess! A friend of mine kept the shirt she covered in various colors, as she rather liked the effect.
That towel backstopped ink refill blunders for the last few years: I can still see the original yellow here and there around all the blotches. Fortunately, the stainless steel sink in the laundry room wipes clean afterward…
Speaking of plotter drawings… I recently made myself an eggbot, which is kind of like a plotter. The firmware is a simple gcode interpreter, which takes instructions from an Inkscape plugin. My question is, do “real” plotters do polygon fills? Is it built in, or does whatever generates the instructions have to draw the fills?
The 7475A can fill rectangles and pie chart wedges with all manner of stripes and hatches, which was just the ticket for bar and pie charts. It could even put tick marks on graph axes, which must have sent shivers down the spines of all those proto-MBAs…
Imagine hand-coding a pie chart in raw HPGL, though: makes Postscript look downright eloquent.
Alas, general polygon fill lies beyond its grasp; the command set can’t define a “polygon” as a distinct entity for further consideration. I bet polygon fill would smack hard against the limits of what little memory they could afford back in the early 1980s.
I suppose you could stick a Raspberry Pi under the covers and run Inkscape as a front end…