Once again I was solving a computer problem for my parents-in-law ;-). This time, it was yet another new printer they had bought – a Canon MG7100. Usually, I have had a really good experience with modern Ubuntu and popular printers. They Just Work. And this time it seemed things had gone well again. And they almost had. Except the colours were a bit off. On the Ubuntu printer test page Magenta was brown, bright green was instead a darker green, and yellow was very muddy – more like taupe. I wasted a lot of time cleaning ink nozzles etc etc but the actual solution was to choose a slightly different driver manually from the Canon list. There were two v4.0 options and it was the second that worked.

Hope this helps someone else.