Canon Multifunction MP540 printing oddly

[#1]

This morning I scanned and printed off something so I could take it along with me when I went to buy some cheap reading glasses. The original document's black was not intense, but a rather  paler shade a little on way to grey. The output had a green shade to it.
This evening I did a deep clean and a nozzle check. The nozzle check showed that The lines that should show magenta shades were absent, although magenta cartridge is listed as half full. A scan and print off this time gave a good rendering of black, but all the red in the original had become green.
Has anyone had that problem before, please? How was it solved.

[#2]

I think a damaged print head can cause colour faults. Does this printer have separate colour cartridges and print heads?

[#3]

Yes, it does. But now here's the funny thing! Just now, for no logical reason, I rescanned the document and all has printed normally, as has a  screen grab of Google colour wheels. I did not turn either computer or scanner off overnight too. Throughout the lights that each cartridge has if correctly working, have been illuminated, bar that for magenta cartridge.
Do those later events tell anything?