Each of palette colors (in 8bpp mode) you can change. For it you need to move frame-cursor to one of colors and press 'Left Softkey' with text 'Edit'. Edit-color-mode window will be opened, when you can setup R,G,B components values for this color (from 0 to 255 for R,G and B). You can apply color changing and return to palette window by pressing 'Ok' ('Right Softkey').