Starting with Magick.NET 7.0.4.100 (just released) you can do this:
MagickColor fillColor; using (MagickImage image = new MagickImage("Waffle.jpg")) { image.Scale(new MagickGeometry("1x1!")); using (PixelCollection pixels = image.GetPixels()) { ColorRGB average = pixels.GetPixel(0, 0).ToColor(); fillColor = average.ComplementaryColor(); } }