The 216 "safe" color palette
Graphic files in GIF format are limited to a 256-color palette. But there are only 216 colors that are common
to all Web browsers. These colors are referred to as the "browser-safe palette" because using them is the
best way to insure that the colors of your graphics are rendered the same on every browser.
Here is different presentation of the "safe palette" which includes the hexadecimal values for each color.
Dithering is a technique for making an image appear to have more colors in its palette
than it actually does. The colors of nearby pixels are changed to make the eye see them as a third color.
A side effect of this is that the graphics have a grainier, speckled look. If you web graphic uses colors that
are not part of the 216 "safe palette," chances are that your monitor will dither to approximate the image's color.
Color on the Web links
RGB Color Chart