Emphasis

Emphasis makes one element the focal point. In the example, one orange tile among blue ones draws the eye at once.

This works because of the Von Restorff, or isolation, effect: when several things look alike, people notice the one that is different, and it can be easier to remember.

Contrast creates a difference between elements; emphasis uses that difference to make one element the most important. Save it for the thing people most need, such as the main action on a screen.