I decided to put my answer in code and comments. Feel free to ask if you need more info.
/* specifying the density befor reading will resolve your size issue */ MagickReadSettings settings = new MagickReadSettings() { Density = new Density(96) }; using (MagickImage image = new MagickImage("34667.eps", settings)) { /* first create a color (make sure it is CMYK when your input image is) */ MagickColor color = new ColorCMYK(0, Quantum.Max, 0, 0); /* the following constructors will be available in the next release */// color = new ColorCMYK("#45D10000");// color = new ColorCMYK((Percentage)27, (Percentage)82, (Percentage)0, (Percentage)0);/* this works for me */// image.Clip("Pfad 1", true);// image.FloodFill(color, 0, 0);/* but this is better */ image.Clip("Pfad 1", false); using (MagickImage background = new MagickImage(color, image.Width, image.Height)) { /* the clipping part makes sure only the pixels you want to be changed are changed */ image.Composite(background, CompositeOperator.In); /* this will work in the next release */// image.Composite(background); } image.Write("output.png"); }