Stupidity, that is.
My printer will not print if one of the cartridges is low or empty, not even in black if a colour cartridge is low. Mine had a low cyan level. Off I go to get the refills and I top up the cyan. Insert the cartridge and go through all the printer diagnostics and there it is, working. I print off a job - letter, black - and all is well in the world.
Back to the kitchen to put the refills away and guess what, I have only topped up the cyan with magenta!
Stupid is as stupid does.