A lot of the platforms out there on the Internet that are browsing your pages are limited to supporting 256 colors simultaneously. And some of those color are already taken up by screen backgrounds, window frames, button highlights and the like. To get the colors specified in your backgrounds, images and text each browser tries to get as close as possible by using approximate colors or dithering [a technique whereby pixels of various colors are placed in close proximity to merge optically and produce a blended color].
A partial solution to this problem is to use colors from a restricted palette wherever possible. One such palette is the Browser Safe Palette. It is a set of 216 colours that are typically rendered exactly as requested by most of the browsers.
As you probably already know colors for browsers are specified using an 24-bit technique often called RGB color. Red is specified by an octet [8-bit byte] that can of course vary from 0 [no red at all] to 255 [as much red as you can get]. Green and blue are specifed in the same way. And because 3*8=24 the resulting specification takes up 24 bits - hence the name. Using hex codes which are the standard way of explaining color choices to browsers in html the values for each color component vary from '00' to 'FF'. The format for the html color specification is 'RRGGBB' [2 hex digits for red, two for green and finally two for blue]. Thus black is '000000' [red, green and blue all completely off] and white is 'FFFFFF' [red, green and blue - each full on]. Red is 'FF0000' and cyan [a combination of blue and green] is '00FFFF'.
The Browser Safe Palette allows six values for each of the color components. In hex these are '00' [0 in decimal notation], '33' [51 decimal], '66' [102 decimal], '99' [153 decimal], 'CC' [204 decimal] and 'FF' [255 decimal]. As you can see the values step evenly over the range from full off to full on in five steps. With 6 choices for each of the three color components [red, green and blue] we thus have 6*6*6=216 different combinations.
On her website Lynda Weinman, who writes about web authoring, discusses this subject in more detail. She also provides two gifs that display these palettes. We have copies of them here. One is organized by hue and the other is organized by value. We also provide three html pages that produce these colors as background colors in table cells. Although current versions of Netscape and Internet Explorer support this html capability not all browsers do. One page organizes the colors by brightness , another by saturation and a third by hue .