Details matter, so without answers to my earlier questions, plus photos, I can only speculate on your problem(s).
Colour contamination seems to come from one of 2 things: either there is residual ink buildup on the printheads and/or carriage, or there is an air siphoning issue.
The first case comes ink gunk built up on the service station area. Some gets picked up by the printheads/carriage and dragged across the paper. It makes a real mess, usually sticky globs/smears of ink on the paper, particularly on the right-hand edge, and sometimes on the back of the paper as well if it gets onto the rollers. It can appear anywhere on a print, not just the leading edge.
The thread you linked suggests to me this isn't your main issue. But it can happen to all Z-series at some point so occasionally cleaning the service station area is always good prevention. Doing this thoroughly is best done by removing the right-side cover, because then you can also access the capping/priming station. The capping station is normally retracted when the carriage is out of the parked position, so you need to manually un-retract it. It's generally not as dirty as the "ink dumping plate" you normally see (I forget it's proper name...), but worth a clean if it's not been done after a lot of use. Photos when I cleaned my Z3100:
https://forum.luminous-landscape.com/index.php?topic=143162.msg1258563#msg1258563So the second possibility, which sounds more like your case, is likely air related. This is why my question about how long the heads have been out of the package is important. If the heads have been in the printer for some time (and particularly if you've done the "clean printheads" option a number of times), then the heads likely have air in them. Easily confirmed by weight -- a full head weighs 60 g. More than a few grams below that means there's air in the heads. The head is not "bad", it just needs to be reprimed with ink to remove the air.
However, if instead the heads are brand new out of the package and this still happens, then I suspect you have air in the ink delivery tubes in the printer, and/or the seals between the printheads and the carriage aren't sealing well and are admitting air. That air can end up in fresh brand new printheads in a relatively short amount of time, so that you have 2 problems: air in both the ink tubes and the printheads.
Then when the head is parked on the capping/priming station, a siphoning action allows a small amount of one colour to contaminate the nozzles of the other colour (at least this is my current theory based on collected experience). That's why the contamination only appears at the start of a print, and typically fades away to normal very quickly.
So the fix here requires getting the air out of the printheads themselves by re-priming them with ink, as already mentioned, and also removing the air from the ink lines in the printer, plus ideally doing something with the printhead seals. There are several threads on here already discussing this in detail, but you can start with the video on LPS Computers for their air purge kit:
https://www.lpscomputer.com/collections/designjet-z3200-parts/products/updated-printhead-recovery-kit-for-hp-70-72-73-91-744-746-771-printheads They've updated their kits/info within the past year to more completely reflect what is known and how to fix it. In short you use a syringe to draw air + ink from the printer lines, then use that same ink to re-prime the printheads and draw air out of those. They also have add-on seals to hopefully help reduce future air admission.
Alternatively, you can just get in the habit of burning a "warm-up" print before every session (especially a calibration/profiling session), but that's costly and time consuming. And the air problem doesn't go away, it only gets worse... After too much air gets in the heads there will be increased cross-contamination, ink delivery issues ("running dry"), and the thermal heads will fail due to overheating (trying to "print air"). These things are discontinued so you need to conserve what you have if you want to keep using it.
If you've never done any of this servicing on your printer, then I suggest start there as it's likely needed. Get a purge/prime kit (from LPS or elsewhere), remove any air from the printer lines, reprime the heads, add seals, and also separately clean up the service station.
