Dithering occurs when you try to display a color on a monitor that is not capable of displaying that color, resulting in a "speckled" appearance. A dithered color is an approximation of a color, that is made by combining pixels of two colors that are available. This happens most often on the web when webmeisters use colors that fall outside the 216 color web safe color palette, and those colors are viewed by a person using a 8-bit (256 color) monitor.            Excerpt from Douglas R. Jacobson