Ok so here is the update. I got my hands on a DESPI-C73 adapter from Good Display (https://buy-lcd.com/products/epaper-hat-connection-adapter-board-for-073-inch-e-ink-display-despi-c73-648) and hooked it up - Same eink panel, same esp32 dev board, same code, same voltage (3v pin of esp32) and same usb power supply - and it justed works flawlessly now
No more green/yellow tint - instead crisp and fully saturated colors.
So all things considered, using the Waveshare 7.3 e-Paper HAT seemed to be the one and only issue here.
After replacing it with the DESPI-C73 adapter from GoodDisplay, fullscreen images with dithered colors are now displaying correcly when powered with 3v
Before the DESPI-C73 arrived, I tried various setups powering it with 5v, since all the symptoms really did point to a issue with the power supply, but none of them helped.
There where some issues getting a error free communication going with the 3v mcu and the 5v eink vin supply, but even after solving them, and trying multiple power supplies (from different USB Chargers to Lab Power Supply, powering mcu and eink together or seperatly, even powering the esp32 and eink with the maximum allowed voltage of 3.6v) the color of the displayed image sadly still had that yellow/green tint...
I also measure the output of the RT9193-33 linear voltage regulator built into the Waveshare 7.3 e-Paper HAT while powering the thing with 3v, but did not find any significant voltage drop there.
Might be that it was just a bad batch, or a discarded revision or clone that is sold on AliExpress (my bad then), hard to tell.
I'll consider the issue to be solved for now, and thank you again Jean-Marc (@ZinggJM ) for your invaluable help - I'm pretty sure I wouldn't have been able to narrow it down to the adapter without you!
Cheers and greetings from Basel 
Christian