I agree with others that the three Epson black inks are not neutral. I'd have said warm rather than green, but it may depend on which generation Epson inkset you're using. You can see this by examining the "warm" curves that ship with QTR which use only the thee black inks without any colors added for toning effect.
Both ABW and QTR have to add in small amounts of the color inks to achieve a neutral print. Unlike Mr Rodney, in my experience neither of them is particularly successful. I find ABW prints at the neutral setting to be a bit cooler than neutral, and for this reason I can usually spot a "neutral" ABW print in an exhibition. QTR neutral is a bit warmer than neutral.
You can tweak both approaches to achieve a more neutral print. It's easier to do this in ABW because there is a color toning wheel in advanced color settings, but it's still trial and error process as the on-screen simulation isn't particularly accurate. Adjusting the toning of an ABW a QTR curve is an advanced process and most users would be better advised just to combine two curves.
The added complication, if you're after exact neutral, is the impact of the paper base color. You may need to tweak for each paper. Which is one of the complications of Piezography K7 inks. Many people don't find their neutral inkset as neutral as they claim, even on Hahnemuhle Photo Rag, but with K7 you can't tweak, other than to change to a different paper.
The Inkjetmall solution to this is their new Pro inkset, which has both warm and cool tonings that you combine to achieve various effects. So you could achieve an exact neutral, although it may again be trial and error for each paper. And you lose part of the attraction of Piezography, as you are reduced from seven to four shades on most printers. But at least you can tweak
I think exact neutral is over-rated and boring. Find a subtle toning that you like.