Since you aren't aware that in a large format printer like this the ink cartridges are not near the head, you probably shouldn't be messing around trying to clean this printer yourself. Whatever you read was most likely for a low end inkjet printer, where the nozzles themselves are part of the ink cartridge.
The z3100 employs a system that records bad nozzles when it is unable to clean them, and remaps those nozzle to "spare" nozzles. Once all of the spares are used up, the printer can no longer fix the problem by cleaning, and the head should be replaced.
It sounds very much like your problem. There are actually 6 "heads" in the printer, each one providing 2 of the colors. The two colors you mentioned do not share a printhead, so it sounds like you may need to replace 2 of them. I believe they cost around $70 each.
Here are the color combos and HP part numbers for the various replacement heads.
HP 70 Light Magenta & Light Cyan Printhead C9405A
HP 70 Magenta & Yellow Printhead C9406A
HP 70 Photo Black & Light Gray Printhead C9407A
HP 70 Blue & Green Printhead C9408A
HP 70 Matte Black & Red Printhead C9409A
HP 70 Gloss Enhancer & Gray Printhead C9410A