An interesting find by the Google Operating System blog: using Google image search to find pictures with a certain color. It is not an official search option (yet), you need to adjust the Google URL manually:
Finding red birds:
  • Enter "bird" in Google image search
  • Add "&imgcolor=red" to the Google URL so it looks like this: http://images.google.com/images?q=bird&imgcolor=red
  • Replace "red" with "blue", "green", "teal", "purple", "yellow", "orange", "pink", "white", "gray", "black" and "brown" to get other colors
Still, watch out for possible copy right violations or poor image quality when using pictures taken from Google in your PowerPoint presentations.
Thank you Ashish for pointing this out to me.