Well, I now have to revise my last post!
As I mentioned, I could print but only in black and white. I checked the brand new colour cartridge and also replaced it - still black and white only.
I decided to go back and start all over again. I deleted all the various drivers which I had tried using AppCleaner to ensure that if there were any odd associated files lying around they would be deleted as well.
I then had a thinking session, a rarity with me according to my nearest and dearest.
I was pretty sure that, on the other old Snow Leopard iMac mentioned in the OP, I had installed a driver from Gutenberg so I went back there and again downloaded usbtb-1.0.17.uni.dmg and installed that.
Maybe I didn't look around hard enough or maybe I just overlooked the vital step, but I found the HP DeskJet 840C driver option, selected that and I am now printing in colour again!
OK, here's what I did this time around for anyone else having the same, or perhaps similar, problem.
Download usbtb-1.0.17.uni.dmg from Gutenberg on
this page. using the signpost information on
this page. this page.
Install it and then find the correct driver, in my case
HP DeskJet 840C - Gutenberg 5.2.3. Follow this path:
System preferences > Print and Fax > Options and supplies > Driver > Print Using > HP DeskJet 840C - Gutenberg 5.2.3
And that did it. Apologies if my post immediately above misled anyone but, as HP have seemingly never made the 840C according to their current and unsupported printers, I think the chances are slim!
