HP 815MFP - Manual (Page 105)

hp designjet 815mfp & scanner 4200 99
tell me about advanced gray balance
This facility is available from Setup > Printer > Add Media Profile >
Calibrate RIP > Advanced.
advanced gray balance
The Advanced Gray Balance calibration is necessary if no
satisfactory standard Gray Balance can be found. While the
standard Gray Balance adjusts the shape of simple gamma curves,
the Advanced Gray Balance can adjust the RGB curves to any
shape via the LUT file.
When doing an Advanced Gray Balance calibration, the approach
is to twist the red, green, and blue curves iteratively, so that the
entire printed gray scale appears neutral to the human eye.