- From an image. Good images have a natural harmony of colors. There are great online tools that allow you to upload an image and extract colors from it (kuler, colr, etc.). This image could then feature prominently on the title page of your presentation.
- From a corporate logo. Many companies have not installed PowerPoint templates with the proper corporate colors. Take a hi-res image of your company's logo and subject it to a similar process as you would do with an image.
- From a color you like. Pick one color you like/is appropriate and let a tool generate matching colors automatically. For example, move the slider in the color wheel in kuler.
For example, for my own web site I was inspired by the fresh colors of an apple and generous amounts of water.
The resulting color scheme can be viewed here.