Windows was designed around the 8*8*8 = 256 colors. Since Windows reserved 20 colors for system use and Netscape reserved another 13 for its own use, this left 223 colors to work with.

This number has been rounded down to 216 web safe colors for PCs and Macs. The number 216 nicely matches a cube: 6 red x 6 green x 6 blue. These colors are "web safe" because they can be interpreted directly instead of dithered to an approximate equivalent system color.

Most likely, the VGA and X11 Colors are probably even more safe than the 216 web safe colors. The VGA colors are the 16 colors specified by name in the HTML specification at the W3C, so in one sense the VGA 16, plus the orange (#FFA500;) of X11 and CSS 2.1 are the real 17 web safe colors.

Each color has a saturation range:

The values chosen from each color are as follows:

  G 00 33 66 99 CC FF
  B 00 33 66 99 CC FF 00 33 66 99 CC FF 00 33 66 99 CC FF 00 33 66 99 CC FF 00 33 66 99 CC FF 00 33 66 99 CC FF
R 00  x                      B                                      G          
33                x                                                        
66                              x                                          
99                                            x                            
CC                                                          x              
FF  R                                                                      x

See these different views of the web safe colors that I've found around the Web:

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