On really old C250 machines, when you can no longer get consumables, I used a yellow IU on the magenta slot (exchanged chip and broke the key tab) and then ran 80 A3 100% magenta color. You can clearly see the color change gradually from yellow to magenta. I've done that a couple of times and it works fine when you're in a pinch (I believe I posted about it at the time, not sure)...
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