Selective - Creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of Web colors. This color table usually produces images with the greatest color integrity. Selective is the default option. Adaptive - Creates a custom color table by sampling colors from the spectrum appearing most commonly in the image. For example, an image with only the colors green and blue produces a color table made primarily of greens and blues.
Most images concentrate colors in particular areas of the spectrum.
This option ensures that no browser dither is applied to colors when the image is displayed using 8-bit color. Using the Web palette can create larger files, and is recommended only when avoiding browser dither is a high priority. This will produce the smallest files, while the other Dither options may improve the image quality. If Dither is needed, you may want to consider JPEG for the image format. Select the smallest number of colors that produces an acceptable looking image.