On the main toolbar, choose (Material Editor) (the Compact version). If the Slate Material Editor button is visible, you need to open the flyout (click and hold) to access the Compact Material Editor button. Or you can just open the Slate Material Editor and then choose Compact Material Editor from the Modes menu.
The Compact Material Editor has sample slots for viewing previews of materials. When you first view the Material Editor, the material previews have a uniform default color.