Couple of ideas here...
First, could you have the White on Black accessability option turned on? Check in System Preferences... Universal Access...
Second, in System Preferences.. Displays.. color, do you have a custom profile set? If so, try one of the generic profiles.. like Generic RGB for instance..
Third, if you create/log in as a different user, does the "negative" effect persist? If not, it's definitely something screwy in your preferences..
Hope this helps...