The Default Design template looks pretty darn colorless at the outset, but even it has a set of color schemes built into it. In creating your own template, you'll want to look at the color scheme that's applied and the others that are available and come up with a scheme of your own.
You can see a design template's color scheme in the Slide Design task pane; click the Color Schemes link at the top (see the picture). The currently applied scheme is selected amid the scheme thumbnails.
A color scheme applies to the background color and the color of various elements on a slide, such as shapes, shape borders and lines, and title and body text.
As you're working on the slide master (and possibly its title master), use the Edit Color Schemes link at the bottom of the Slide Design task pane to change colors in the scheme. By doing so, you create your own scheme.
Note If you use a photograph or bitmap graphics file as a background, such as a .jpg, .gif, .bmp, or .png file, it will not be affected by your choice of color scheme. A vector graphics file, such as the drawings you find in Clip Art and Media on Microsoft Office Online, will be updated with your color scheme.