On 22/01/2019 19.47, NY wrote:

And how many printers *do* print black text in all four colours? Looking
at a page of black text under a high-power magnifying glass, I can't see
any evidence of coloured ink around the edges of the black text, so it
looks as if my printer just uses black ink for black text - but still
requires all the cartridges to be working even to do that.

Because the colour can be defined as black and thus use only black ink,
or not pure.

If you assign a byte to each ink, you can say a pixel is:

FF000000

and that will print in black only (whatever that ink really is). But it
could be "FF000100" and there would have a tiny bit of yellow (say).

To be sure, on some printers you have to choose "B/W mode". If the
computer sends in that mode, the printer should not use any colour at all.

--
Cheers, Carlos.