code: public void setCustomAreaFillBottom(final Color CUSTOM_AREA_FILL_COLOR_BOTTOM) {
          customAreaFillBottom = CUSTOM_AREA_FILL_COLOR_BOTTOM;
          init(INNER_BOUNDS.width, INNER_BOUNDS.height);
          repaint(INNER_BOUNDS);
      }
docs: Sets the color that will be used for a custom area gradient at bottom
  @param CUSTOM_AREA_FILL_COLOR_BOTTOM