New Post: Reduce pictures to very low number of colors

You can reduce the number of colors with the Quantize method. You can specify your own ordered dither map with the OrderedDither method. But then you will need to use your own .xml files and modify the thresholds.xml file. You can find some information about the file format for the threshold.xml file here: http://www.imagemagick.org/Usage/quantize/#diy_threshold. The xml files can be found here: https://magick.codeplex.com/SourceControl/latest#Magick.NET.Wrapper/Resources/xml/ and the documentation page has an example on how you can initialize the Magick.NET environment with your own xml files.