if (UIManager.getLookAndFeel() instanceof MetalLookAndFeel) {
   MetalTheme mt = (()UIManager.getLookAndFeel()).getCurrentTheme();

   // The following is true for DefaultMetalTheme
   // primary 1
   mt.getPrimaryControlDarkShadow();
   // primary 2
   mt.getControlShadow();
   // primary 3
   mt.getPrimaryControl();


   // secondary 1
   mt.getControlDarkShadow();
   // secondary 2
   mt.getPrimaryControlShadow();
   // secondary 3
   mt.getControl();
}

Looking at the JDK sources might also help.