In a paletted image, when you select a color from the color table or the list view, the Change button becomes active.
Clicking the Change button gives you the option to change the selected color. In the Target Color dialog box, you can select a color from the current palette, or from a standard palette, such as the AutoCAD palette or a spectrum palette. You can also select a custom color.
As with most changes in the Palette Manager dialog box, the changes you make to the palette are not permanent until you click the Apply button.