Domenico, what you refer to are the so-called "web-safe" colours.
[Wikipedia](
http://en.wikipedia.org/wiki/Web_colors#Web-safe_colors):
|> At one time many computer displays were only capable of displaying 256 colors. (..) As of 2011, personal computers typically have 24-bit (TrueColor) and the use of "web-safe" colors has fallen into practical disuse. Even mobile devices have at least 16-bit color, driven by the inclusion of cameras on cellphones.
[W3Schools](
http://www.w3schools.com/html/html_colors.asp):
|> Some years ago, when computers supported max 256 different colors, a list of 216 "Web Safe Colors" was suggested as a Web standard (..) This is not important today, since most computers can display millions of different colors.
Also, if you check the W3Schools link you'll get tables with 32 shades of red and 32 shades of grey. See for yourself whether your monitor can display each of them distinctly. :)
