Grayscale/Greyscale is missing the LUT for 4.2" 400x300 greyscale panel (PID 6381)

Greyscale mode doesn't work as expected when trying to use White with this product: https://www.adafruit.com/product/6381
Mono mode works fine.

I was trying to use fill with white/black, and labels similarly in white or black.

This is what I am now seeing after the patch in #2:

Start by inspecting the LUT handling in the driver and the patch referenced in #2, then reproduce grayscale mode with the 4.2" 400x300 panel (PID 6381). Done means white and black fills and labels render correctly in grayscale mode while mono mode remains working.

Written by the indexing model from the issue text.