This is a bug from the original Mangle that I emailed the creator about but he never replied.
The Kindle 3 (possibly the DXG as well) is using the Kindle 2 colour map which only has 15 colours (missing #EEEEEE)
That should make the dithering look much nicer.
BTW a new version of Canti is out which fixes a few bugs http://www.bitcake.com/?page=project_canti
Make sure you install ImageMagick before trying Canti.
Canti has all of the features of Mangize and Mangle (well minus a good GUI) and more.
You can basically give it an entire zipped series and it will automatically convert it and name each volume like Blah_-_v##_c###-###.cbz/pdf/folders