When converting between RGB and CMYK you should use color profiles. You can do that like this:
You can download the profile here: http://www.adobe.com/support/downloads/detail.jsp?ftpID=4075. That zip file contains multiple profiles. You should probably try to see which CMYK profile provides the best result for you.
using (MagickImage image = new MagickImage("agent-j-peck1Before.tif")) { image.AddProfile(new ColorProfile("USWebCoatedSWOP.icc")); image.ColorSpace = ColorSpace.CMYK; image.CompressionMethod = CompressionMethod.LZW; image.Write("agent-j-peck1After.tif"); }