Web Design in a Nutshell, 2nd Edition

Converting to the Web Palette

Designing with web colors is one method to prevent dithering. The other opportunity is to add (or preserve) web-safe colors in the conversion process from RGB to Indexed Color. As part of this process, you will be asked to select a palette for the image. The colors from the palette will be used to approximate the full color range from the original RGB graphic. See Common Palettes for descriptions of palette options in popular graphics programs.

Common Palettes

All 8-bit Indexed Color images (such as GIF or PNG-8) use a palette of colors to define the colors in the image. The web palette is just one of infinite palette possibilities. There are several standard palettes you can choose from within popular graphics programs.

Exact

If the image contains fewer than 256 colors, choosing the Exact palette option makes a palette out of the actual colors that are found in the image.

Adaptive

This is a custom palette generated with the most commonly used pixel colors in the image. It allows for color-depth reduction while preserving the original character of the image. Because the number of colors is being reduced, some dithering and color-shifting will occur.

Perceptual (Adobe Photoshop/ImageReady only)

This creates a custom palette by giving priority to colors for which the human eye has greater sensitivity. Unlike Adaptive, it is based on algorithms, not just a pixel count. It generally results in images with better color integrity than adaptive palette images. ...