This is not possible with the current release of Magick.NET. This is the code that you can use in the next release of Magick.NET:
using (MagickImageCollection images = new MagickImageCollection("bunny_bgnd.gif")) { foreach (MagickImage image in images) { image.ColorFuzz = 5; } images.OptimizeTransparency(); images.Map(); images.Write("bunny_bgnd.new.gif")) }