This control lets you edit each palette slot in the palette associated with the current image. Deep color images don't have a palette and the Edit Palette option is thus disabled. In this case you can use the Convert to option to convert the image to 256 colors or fewer.
Select the color you would like to edit by clicking on the desired color slot. The slots are ordered from left to right, top to bottom with the first slot at the top left corner. The palette slots are arranged in order of frequency. The first slot contain the color that has the highest frequency (is most used) in the image. The last slot (bottom right) has the lowest frequency.
If the image has fewer than 256 colors the empty palette slots are disabled (indicated by a diagonal line pattern). These empty slots cannot be edited.