Each batch of displays has its own unique waveform tables, which vary by production run and by temperature. These are programmed following a long series of calibration tests using representative samples from each batch. The waveform data in the ROM on the display cable must match the data in the display, and some replacement displays come with NO waveform data in the ROM. In addition, there should be a voltage (between -0.3v and -2.5v) written on (or near) the cable, and that the VCOM voltage must be adjusted on the kindle motherboard to match that written value (see image below), for the best, most accurate, greyscale values on the display. Some eink devices have an adjustable potentiometer that should be adjusted to match what the replacement display requires. Other (more sophisticated) devices may have a software-adjustable voltage setting.
If the waveform tables OR the voltage are "off" for that display, it will not display accurate greyscale values.
This is GENERAL information for how these eink displays and eink controllers work (based on the early eink dev kits), but specifics may vary by kindle model in our case.
There is also a thread here dedicated to hacking waveform files to work with replacement displays:
https://www.mobileread.com/forums/sho...d.php?t=259903
https://www.mobileread.com/forums/sho...98#post2952198