Here's the '256 shades - test pattern' rendered by PDFRead with the modified NRhtml2mobi.exe using 4, 8, and 16 colors.
I don't know if the 32, 64, 128 and 256 color versions will look any different...
I also got a "full color/grayscale" version using 2 colors (I guess it's a bug with PDFRead).