Dithering

• a technique for varying the patterns of dots in an image in order to give the impression of shades of grey. Each dot, however, is of the same size and the same intensity.
• to create the illusion of color depth in images with a limited color palette (quantization) 
• colors not available in the palette are approximated by a diffusion of colored pixels from within the available palette 
• analogous to the halftone technique used in printing

An illustration of dithering. Red and blue are the only colors used, but as the pixels become smaller, the patch appears magenta.

Dithering is the attempt by a computer program to approximate a color from a mixture of other colors when the required color is not available

• Eg. color is specified for a Web page that a browser on a particular OS can't support. The browser will then attempt to replace the requested color with an approximation composed of two or more other colors it can produce. The result may or may not be acceptable to the graphic designer. It may also appear somewhat grainy since it's composed of different pixel intensities rather than a single intensity over the colored space