You can invert the colours of the photographs stored in your iPhoto library, but it is not an official feature, so will require a counter-intuitive process to pull it off. However, when you finish, the result will be a stunning and bizarre graphical effect that simulates the look of photographic negatives.
Right-click and select "Show Original File."
Right-click the file and select "Open With Preview."
Select "Tools" and "Adjust Colours."
Move the black slider all the way to the right. Move the white slider all the way to the left. This has the effect of inverting the colours in the photo.
Press "Cmd" and "S" to save.
Return to iPhoto, right-click the photo and select "Revert to Original."