HP LaserJet 3600 printing wrong colours for specific shades

[AngryTechnician]

In a continuation of a long-running tradition of me despising any computer peripheral beginning with P (printers, projectors, Promethean boards... OK, I cheated slightly on that last one), one of my wretched HP LaserJet 3600 printers has been annoying me today as it has started printing specific shades of colours completely incorrectly.

 

In printouts of photos, this shows up as blotches of an odd colour where it shouldn't be, usually a magenta blotch. in the middle of some green or blue. I've ruled out any driver or software problem, as the colour sample prints that you get from the print menu itself are also showing up problems. Here's the swatches from overview page:

 

(Click to enlarge)

 

 

And here's the detail from page 15 of the detailed swatches. All of these should be a shade of blue:

 

 

The fact that is is very specific shades leads me to believe it is not a consumable problem either.

 

Have tried cold reboot, factory reset, and reseating the formatter board. Anyone seen anything like this before or have any idea how to fix it (other than my preferred HP hardware fix of defenestration)?

[AngryTechnician]

OK, turns out I had forgotten that on HP kit, FACTORY RESET actually just means 'reset options to defaults'. Finally remembered where the COLD RESET option is (in the boot-up menu you get by holding down the go button at power-on) and that did an proper factory reset, which seems to have sorted it, at least for now.