Sometimes a MacOS X computer will have a strange appearance on the monitor. This document lists possible causes and cures.
The setting Switch to White-on-Black is meant to assist people with certain types of visual impairments. However, it seems that many students just find it fun to use. If the monitor looks “inverted” or resembles a picture negative, this has probably been turned on.
White-on-Black is turned on and off by the keystroke <Control>-<Option>-<Apple>-8. Press the 8 key last and release it first, just as you would with <Shift>-A to type a capital letter.
This setting can also be changed by clicking on the Apple Menu, selecting System Preferences, selecting Universal Access, and selecting “Black on White” under the Display Menu.
If the screen is completely devoid of color, and everything appears as black, white or grey, this is often due to Greyscale being selected.
To fix this, click on the Apple Menu, select System Preferences, select “Universal Access”, look under the Display menu, and uncheck “Use greyscale”.