GIFs are limited to a palette of 256 colors. If a graphic is converted to the GIF format, colors that lie outside of that palette are approximated using a process called dithering. In the dithering process, alternating pixels of similar colors approximate the original color's appearance. Dithering can cause GIFs to appear with jagged edges and fuzzy colors. To avoid dithering, you can limit your GIFs to only use colors from the safety palette.

The safety palette is a collection of 216 colors that are guaranteed to be displayed without dithering. The palette limits the red, green, and blue components to the values: 0, 51, 102, 153, 204, and 255. Thus a color value like (204, 51, 153) would be part of the safety palette, but (192, 124, 255) would not. The tables below show all of the colors of the safety palette along with the values of their red, green, and blue components. Note that the safety palette is only required for GIFs. Other graphic formats such as JPEG and PNG support higher color resolutions and thus will not have to employ dithering to display colors outside of the safety palette.

Carey, P. New Perspectives on HTML and XHTML 5th Edition, Course Technology: Boston, 2008.