Crosshair that inverts color underneath?

@Kalle_H Okay, so I found the sceneColor node – it’s in the material editor. So I can make a material that is the opposite of the scene color.

How do I apply a material to an image widget? (is there some other widget I should use for the crosshair other than “Image”?)

From what I can tell there is nowhere to select a material for an Image widget:

Screen Shot 2017-01-05 at 4.40.31 PM.png&stc=1

Am I missing something obvious?