Some time ago I wrote this post HP Color LaserJet MFP M277dw: previews and prints in colour instead of grayscale. Today I came back to the topic after a while and made some new research.
I haven't really found a clean solution, that is how to tweak the ordinary settings to the desired effect.
However, I discovered that I can fool the computer, install the driver of a sibling printer and then have the colour/greyscale toggle functioning again. I would call this a workaround, indeed. A sort of trick, say...
In a way this is a sort of answer, but not quite the one I myself aimed to. The fact that I would be answering my own post makes the situation only a bit more peculiar, but I don't believe it's essential.
Do workarounds qualify as answers or rather as comments? To which extent is this a point of linguistics?
NB Obviously anyone is welcome to post a clean answer to that thread... and a workaround too?