From any slide in the presentation, open the Format menu and select Slide Design. Or, select the Slide Design - Color Schemes task page.
Click the Color Schemes option on the Slide Design task pane.
Scroll through the available slide colors and click the one you like.
The color is applied to all slides in the presentation that have not been previously assigned a specific color (and the text and objects remain intact).
INTRODUCTION
In addition to offering numerous templates, slide layouts, and slide designs, PowerPoint lets you alter the color schemes of your slides. Some slide designs have only a few colors formats from which to choose, whereas others have several. If you are feeling adventurous, you can always edit the color schemes to your own liking.
TIP
Applying a Slide Color Scheme to Specific Slides
To apply a new slide color to specific slides in your presentation, press the Ctrl key while clicking each slide (in the Slides tab). When you move the mouse pointer over a slide color in the Slide Design Color Schemes Task pane, click the down arrow and choose Apply to Selected Slides.