New Post: Composite Multiply Gray Scaled Images

using (MagickImage mask = image.Clone())
{
    mask.ColorSpace = ColorSpace.HSB;
    mask.Negate(Channels.Green);

     using (MagickImage newMask = mask.Separate(Channels.Green).First())
     {
         using (MagickImage maskBlue = mask.Separate(Channels.Blue).First())
         {
             // Becomes a RGB image.
             newMask.Composite(maskBlue, CompositeOperator.Multiply);
         }
      }
}