Old fashioned transparencies, often referred to as overheads, worked much better as dark text on a light, preferably white background. Modern computer slide show projectors use a different technology. With slides, light colored text works best against a dark background.
What background color is best? Some authorities suggest trying to match colors to the emotional mood you are trying to create. For example:
Purple: Royalty, wisdom, spirituality, mystery
Green: Nature, environment, health, reptiles, insects
Gray: Conservative, practical, reliability, security, staid
See the Think Outside the Slide website for more.
Certainly there might be some benefit to this approach in some situations. However, as a practical matter I usually give these factors little consideration when preparing my own slide shows. I normally use the color combination that is generally acknowledged to have the highest legibility: Dark blue background, with light text, usually white or yellow. The most important thing for me is that the audience be able to read the slides easily. I’ll use methods other than color if I feel I need to maniuplate the audience’s emotions.
Whatever color scheme you choose, to improve legibility try to maximize the contrast between foreground and background. Light grey text on a medium gray background is a recipe for disaster. The Think Outside the Slide website has a color contrast calculator.