Thanks for your helpful input tjncooke.
Yes, dampers and other parts of the ink system have nooks and crannies for color pigment to get stuck, and cause color staining as it slowly gets flushed out. The amount of pigment build up depends on the age and use history of the printer. Older machines, or printers that have sat unused for periods of time are likely to have more build up. If the printer is over 3 years old, it's recommended to replace the dampers anyway, and new dampers would quickly eliminate the color staining you may be struggling with. In this printer model, the ink system is sold as one whole unit, which is how the repair manual instructs to replace the dampers, though we and some customers have successfully replaced the individual dampers, that is dependent on one's technical and confidence level, and is a "proceed at your own risk" procedure, as there are no formal instructions.
I have great success purging individual lines by printing the QTR purge sheets, though depending on the amount of pigment stuck in your dampers, you may need to print pages until they come out pink, then let the printer sit overnight (to let more color ink seep out), and print some more purge sheets, then repeat until no more color comes out.
This is also true when doing cleaning cycles. The dampers themselves hold very little ink- only a few ml, but you need to pass more fluid thru to flush out the crevices where color can get stuck.