I found this thread with an example BMP using 8 bits per pixel:
https://www.mobileread.com/forums/sho...d.php?t=129623
It seems the eips on Kindle 3 Keyboard understands 8bpp (256 colors) although it can only display 16 colors.
This seems to work as well, though with 4 bits per pixel the filesize is only about half the 8bpp bmp and thus faster to write.