A color gradient is a visual effect in which a solid color gradually changes from light to dark or dark to light. PowerPoint offers several gradient patterns, each with several variations. You can also choose a preset arrangement of colors from professionally designed backgrounds in which the different colors gradually merge.
If you want something fancier than a gradient, you can give the slide background a texture, or you can use a picture. PowerPoint comes with several preset textures that you can easily apply to the background of slides.