No idea, if you have that printer print a neutral greyscale stepwedge, take a microscope and check whether there is color ink used to make the print neutral. If not and you consider the print neutral then the 3 Grey inks are neutral. As far as I know the HP Zmodel have neutral grey inks as the only wide formats and will lay down only grey inks when they get a neutral B&W image to print.
Thank you very much, Ernst.
No, I'm not an 7900 owner. I have to decide which printer to buy between the x900, Canon ipF and HP Z3200 series.
I think that neutral grey inks are better manageable in BW printing, because of less coloured inks needed to reach the desired tone (neautral or cold). This can determine an higher lightfasteness and less problem due to pigment settling.
The 7900/9900 grey inks color can be observed in a nozzle check test or similar ....
... even better printing a 21-step chart from QTR, selecting ONLY a grey channel.