Thanks for your inputs Phil. Blocked and deflected nozzles - sure, no-brainers. But I would sincerely appreciate it if you could educate me as to how to recognize pigment separation, significant ink delivery issues, head vs. ink lines problems, and so on by studying the nozzle check print.
The nozzles of my printer are not blocked and not deflected. As for a reduced ink delivery of a particular color, say a reduction of 10 to 30%, I don't see how you would recognize that by looking at the nozzle check print, but it sure would cause a hell of a color cast like the one I'm experiencing.
Sure! Pigment separation is evidenced most easily with a blue light/UV lamp, but a loupe can also be enough - you're looking for the clear carrier being ejected without any pigment - so it looks like a blockage or partial blockage but in fact you have liquid hitting the page. If you can see that the entire line of the nozzle pattern is "wet" but there's no colour, or if you have limited amounts of colour (either patchy or reduced intensity), then you are most likely looking at pigment separation (so check the carts are not massively out of date or more than 12 months since opening, and if not then agitate them and run a clean to move good ink into the lines again).
Similarly, when you're looking carefully at the nozzle checks, or more specifically at a series of them before and after printing, you can see if there's an ink delivery issue through the reduction of liquid in varying places that gets worse after printing. Head versus lines? Do the blockages move or not, do they clear and then come back? True head blockages or head failure will result in missing nozzles that don't change (note that there might be a few peripheral nozzles "moving" but a core of unchanging blockages is what you're looking for here). On that same note, if you have an entire colour missing that won't recover but all others are fine, it's probably a significant supply issue because it's very unusual to have that occur due to localised blockages or electronic failure (electronic failure will either give you incorrect output during printing or no output across the whole head (rarely you can have other results).
There are other things you can discern, too, but you need to know which printer, which media, and a bit more history (age of unit, environmental conditions, and so on). Often you need a series of checks and test prints. Sometimes, though, you need to physically check lines, dampers, and the like, but you try to check what you can without pulling the unit open and that's where nozzle checks and test prints come into it. It takes time to recognise these things quickly, but anyone can make a good assessment given a bit of time and some tools to help them look more closely.