Which version of PowerPoint do you use? If those slides are based on a slide
master, you can change the color of the placeholder text in the master. If
not, you should find someone, who writes a small VBA macro to do it.
If you get back with EXACTLY what changes you need I'll post the exact code
(make sure you say whether text is in placeholders, textboxes or both and
whether you want to swap ONE colour for another or ALL colours to one colour)