Who owns the color of the edges of a Java Swing scrollable viewport?

I have a JScrollPane with a JPanel that implements Scrollable as its viewport view. When I set ScrollableTracksViewportHeight and ScrollableTracksViewportWidth to false, my panel stays at its preferred size. Good. The problem is that I can't figure out who owns the space around the scrollable panel. I feel like I've tried changing the background of every component and dialog, and nothing seems to do it. How can I change the ugly grey to a fun color like neon yellow?

Here's a screenshot of what I'm talking about:

Here's what I want:

And here's the code to duplicate the first image:

package components;

import java.awt.*;
import javax.swing.*;

@SuppressWarnings("serial")
public class DialogWithScrollPane extends JFrame {

  ScrollablePanel scrollablePanel;

  public DialogWithScrollPane() {
    super();

    setResizable(true);

    Container pane = getContentPane();

    scrollablePanel = new ScrollablePanel();

    final JScrollPane scrollPane = new JScrollPane();
    scrollPane.setPreferredSize(new Dimension(300,300));
    scrollPane.setViewportView(scrollablePanel);
    scrollPane.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
    scrollPane.setHorizontalScrollBarPolicy(JScrollPane.HORIZONTAL_SCROLLBAR_ALWAYS);
    pane.add(scrollPane);

    setMinimumSize(new Dimension(300, 300));        
    setVisible(true);
  }

  class ScrollablePanel extends JPanel implements Scrollable {

    public ScrollablePanel() {
      super();

      setBackground(Color.red);
      setPreferredSize(new Dimension(200, 100));
    }

    public Dimension getPreferredScrollableViewportSize() {
      return getPreferredSize();
    }

    public int getScrollableBlockIncrement(Rectangle visibleRect, int orientation, int direction) {
      return 0;
    }

    public int getScrollableUnitIncrement(Rectangle visibleRect, 开发者_JS百科int orientation, int direction) {
      return 0;
    }

    public boolean getScrollableTracksViewportHeight() {
      return false;
    }

    public boolean getScrollableTracksViewportWidth() {
      return false;
    }

  }

  public static void main(String[] args) {
    javax.swing.SwingUtilities.invokeLater(new Runnable() {
      public void run() {
        new DialogWithScrollPane();
      }
    });
  }
}

scrollPane.getViewport().setBackground(Color.yellow);

It all belongs to the JViewport, which you can paint any desired color, as shown here.