This is a computer mock-up of it, since it's hard to see in a photograph or scan:
It's two colors: a dark gray and a green. From a distance, it looks completely black. As you approach it, you see the image and the two holes at the top... (photos from the Tate Modern)
...and then the green peeking through the gray. (a scan of the print)
A couple sketches from it:
