You should call the Write method of the WritablePixelCollection class that is returned by image.GetWritablePixels() to make sure the pixels are written to the image. You could also do what you are doing in a different way:
using (MagickImage image = new MagickImage()) { image.Read(@"C:\.....\test1.png"); image.Evaluate(Channels.Red, EvaluateOperator.Set, Quantum.Max); image.Write(@"C:\.....\test2.png"); }