Using the Palette Manager, you can control the 256 colors or shades of gray in any indexed image. For example, you can combine similar shades to reduce the palette, or highlight some details in the image by blanking out the rest.
As shown in Managing the Image Palette, the array of 256 squares at the top of the Palette Manager is known as the color table. The display at the bottom is an interactive list view. Using the Sort By drop-down list in the upper right, you can sort a palette by index number, frequency, background transparency, color, hue, saturation, or luminance.