This happens on Macintosh computers that can support 16, or 24 bit color. If you select the grays radio button in the Monitors control panel, the Apple will display solid black because of guidelines for displaying the Apple logo.
The reason that the rest of the screen is not in shades of gray, is because in addition to having selected grays, you have also set the control panel to either thousands or millions of colors. Since there are only 256 grays available to the system, everything over the 256 grays is displayed in color.
Support Information Services