These six values produce 216 possible combinations of red, green, and blue, and they will look the same on nearly all computers. See Lynda Weinman's 216-color chart . This chart from the home page of the author of the very good book Coloring Web Graphics gives the colors and their hexadecimal and decimal values. Dithering is undesirable in web page GIF files. The way that GIF files are coded, dithering creates much larger GIF files than simiilar non-dithered ones.
Dithered files are therefore slower to load.
If text using a small font is displayed over a dithered background, it can be a lot harder to read, too. For clean, fast loading web pages, all GIF files should be made up only of the 216 basic colors. Note that GIF and JPEG files use completely different coding schemes, and are used for different types of graphics. You don't need to be concerned about how many colors are used with JPEG files. Aliasing and Anti-Aliasing.