Figure and ground

Figure and ground describes how people separate what they see into a foreground, the figure, and a background, the ground.

In the example, the vase is the figure in both panels. On the left it is dark on a light ground; on the right it is light on a dark ground.

Interfaces use this all the time: a dialog over a dimmed page, or a card on a tinted background. Make the figure clearly different from the ground so people know where to focus.