Well, it depends on how the presentation was set up. You can check the
color schemes pages on my site (URL's in signature) for information on
one easy way to do this -- assuming that the presentation was set up so
that this will work.
Luc's suggestion on the outline pane is a very good one, too. That will
only work if the text is in placeholders, though. If you have manually
added textboxes, those would need to be changed via the color scheme
(depending on how the color was originally selected) or manually.