Symptom
Your see a negative image in the screen. Depending on system configuration, it may be black and white, such as that in Figure 1, or color negative, such as that in Figure 2.
Figure 1 Picture of black and white negative image display using Universal Access
Figure 2 Picture of color negative image display using Universal Access
Solution
You can toggle the display between positive and negative by using the Control-Option-Command-8 key combination. First, press and hold the modifier keys (Control, Option, Command). While holding the modifier keys, press 8.
The screen will go blank for a second and return in the opposite mode.