When working on my design for web brief, I thought that I would look up web safe colours so that I can design around using these colours, rather than designing my website and finding out the colour I want to use isn't web safe and doesn't look right online.
Netscape introduced a fixed color palette of 216 colors that will be used on platforms with a graphics mode with only 256 colors. Other colors will be dithered to that color palette. This means that the browser replaces the colors that are not in the fixed palette by the closest matching color in this palette. If you do not want to the browser to dither the colors on a page you should stick to these colors.