New Post: Combine layers

Hi,
I have one image that is a car.
I need to apply to this image a layer: trasparent for a percentual and trasparent dark gray to other part.
When I try to combine, it's work but the color are diffently.
How I can do it?
I can use any type of image (gif, png ecc.)

My code is

using (MagickImageCollection images = new MagickImageCollection())
{

 MagickImage first = new MagickImage(@"c:\testdati\36.gif");
 images.Add(first);

 MagickImage second = new MagickImage(@"c:\testdati\layer.gif");
  images.Add(second);

  MagickImage risultato = images.Combine();                        
  risultato.Write(@"c:\testdati\risultato"+i+".gif");

}

Thanks.!

Original image to apply
Layer image
Result: