There is something weird in the color rendering of Microsoft PowerPoint 2011 for Mac. First, text and shapes get treated differently. If you make the text and the background the same color, the text will appear different. At first it looks like to be designed on purpose. But the adjusted color is actually a bit off on the hue spectrum, creating color clashes. See the example below.
Maybe there is a problem inside the software though. Look at the screen shot below of a presentation in presenter mode. I copied 2 exactly the same slides and you can see that the preview of the second (identical) slide pulls the blue into same purple direction as the text in the previous example. There must be more than one color rendering engine inside PowerPoint.
PowerPoint 2010 for Windows does not suffer from this, and I hope that Microsoft will fix it in a subsequent update (even it was done on purpose). If I want my text to stand out on a background, I want to freedom to decide myself what colors to pick.