Check your slides in B/W view
In any presentation situation, it's a fair bet that 10% or so of the male members of your audience will be color-blind to some degree. This doesn't mean that they can't see colors at all, but they may have trouble distinguishing certain colors, most commonly red vs. green.
It's a good idea to make sure that your text and other important elements contrast with the background in both color AND value -- that is, that you use light/dark contrast as well as color contrast to make them easier to tell apart.
A good way to check for value contrast is to use PowerPoint's B/W View.
Simply choose View, Black & White (or View, Grayscale if you use a Mac) from the main menu. PowerPoint displays your presentation as it will print to a B/W printer.