Lock or unlock colors in a palette
You
can lock individual colors so that you cannot remove or change them
when changing palettes or when reducing the number of colors in
a palette. If you switch to another palette after colors have been
locked, locked colors are added to the new palette.
To toggle locking for a selected color swatch, click
the Lock button 
at
the bottom of the Optimize panel, or Right-click (Windows) or Control-click (Mac OS)
the color swatch, and select Lock Color.
To unlock all colors, select Unlock All Colors from the Optimize
panel Options menu.
PDF (adobe.com)
