Well, if we look at the behind-the-scenes making of this panel (in film) you would see Mary with her torso painted green, Then that would be superimposed over footage of Phantom Lady mock hugging around her while pressed up against a plane of glass. Wait... how would I get Phantom Lady's arms in front of Mary? A-ha! An additional set of footage with Phantom Lady completely covered in green except from her shoulders to her fingertips. Wow. Movies are hard.