Color can set the mood of a presentation, and its selective use can make things easier to see. Conversely, poor color combinations will set the wrong mood, render an image unattractive, or make it unreadable.
Keep the background color constant across all slides.
Limit colors in most graphics to no more than two or three and stay within the same family of hues.
For typeface and graphics, use colors that contrast rather than clash with or blend into the background; check for visibility when projecting.