Is your slide showing photographs? Diagrams/graphs/charts? Just words? A mixture?
It's going to be a judgement call, but here's a good rule of thumb. Where possible:
Make the background dark, preferably dark charcoal
Charcoal? Why not black?
Because black is too harsh. There's too much contrast. Our eyes don't like it, so it's harder to comprehend the message.
A couple of extra points:
Photographs. With few exceptions photographs should be set directly into dark charcoal without borders. However, if the photograph of an object sits in a white rectangle, you'll need a white background.
Diagrams/graphs/charts. Yes, I still favour the charcoal background, but I'm careful with the colours of the diagram - light and bright, rather than dark and dull. That's for the essential points of the diagram, but I usually use white for the supporting material, such as words and axis. It pays to decide on background before you make up the diagram - you can't drop a diagram made for a white background directly onto a dark background without making some changes.
Just words. There's no doubt. Our eyes take in white words on dark charcoal more easily than any other way - with obvious implications for comprehension.
A mixture of the above. Again, lay it all on dark charcoal.
Now, a word or two of warning.
Beware of words-only slides
All right, sometimes there's no other way. You might, for example, want to display exact wording. But the screen medium suits pictures more than words - whatever the background colour. Show your audience too many wordy slides and their eyes will glaze over, undermining your message. We humans sell ideas more effectively than words on a screen.
Does reading out the words to the audience help? Well, yes, you can justify that if you want to emphasize it in a way not obvious in the screened words. But you should know that when I carried out a survey entitled, What annoys you most about PowerPoint, the top annoyance was presenters who read aloud words the audience can see for themselves.
Beware of wall-to-wall PowerPoint
Here's some bad news for presenters who think that presentation means start-to-finish PowerPoint.
With rare exceptions, non-stop PowerPoint on the screen damages your credibility. Especially when what you're saying doesn't match what's on the screen. Photos, diagrams, or words - I urge you to do yourself and the audience a huge favour. Stand directly in front of the audience, until...
Use a screen only when the image directly and specifically
illustrates what you're talking about right now.
How to turn it off? You might like to look at another blog: PowerPoint tip 10: insert a 'sleep slide'