Web Design in a Nutshell, 3rd Edition

Indexed color (8-bit)

In 8-bit color systems, there are 8 bits of data to handle all the color in the monitor, which means that the monitor can display only 256 (2⁸ = 256) colors at one time. There are also monitors with lower color depths, such as 1-bit (two colors: black and white) and 2-bit color (four colors), but they are fading out of use by the general public.

Indexed color is fundamentally different from the previous two models. Rather than assigning bits per channel, an indexed color system keeps a set of colors (called a palette or color map) that are available to be displayed at any one time. In 8-bit displays, each color in the palette is assigned a number, or an index , from 0 to 255. The color of each pixel in the display is represented by its index number, which then corresponds to the respective color in the palette.

Indexed color palettes consist of colors from the full RGB color space (in other words, 8-bit color is a subset of Truecolor). The 8 bits of data merely limit the number of colors that can be displayed at one time.

Tip

Understanding indexed color in monitors is useful for understanding how color works in graphics in the GIF format, which also use 8-bit, indexed color.

For system-level operations, computers use a specific set of 256 colors called the system palette . Macs and PCs use slightly different sets of 256 colors in their system palettes. Specific applications may use their own palettes; for instance, browsers have a built-in palette, known as the ...