This is a bug in Magick.NET that will be fixed in the next release. You can use the following workaround that will stop working after the next release:
using (MagickImage magickImage = new MagickImage(@"c:\thumb.bmp")) { // Assumption is the mother of all ... magickImage.Modulate(new Percentage(9000), new Percentage(10000), new Percentage(10000)); magickImage.Write(@"c:\thumb-mod.bmp"); }