For what its worth, I resolved the problem on my end by simply installing Ubuntu and printing with stock drivers detected by Ubuntu. It works perfectly. There is a night-and-day difference between what I get from Ubuntu vs Windows.
In ubuntu yellow is pure yellow ink. Magenta is true pure...
I have The EPSON ET-2720. I found this forum because I also cannot get true yellow from my printer.
TLDR: Manufactures make junk drivers for their inkjet printers and won't let you print true colors.
The long:
I was trying to print CMYK documents as I make vector artwork, which is awesome...