Try the following code:

import java.awt.*;
import java.awt.image.*;
import javax.swing.*;
import javax.swing.plaf.synth.*;
import javax.swing.tree.*;

public class TreeBug {
    public static void main(String[] args) throws Throwable {
        SynthLookAndFeel lookAndFeel = new SynthLookAndFeel();
        lookAndFeel.load(TreeBug.class.getResourceAsStream("treebug.xml"),
                         TreeBug.class);
        UIManager.setLookAndFeel(lookAndFeel);
        JTree tree = new JTree();
        tree.setSize(100, 100);
        DefaultTreeCellRenderer dRenderer = new DefaultTreeCellRenderer();
        tree.setCellRenderer(dRenderer);
        BufferedImage image = new BufferedImage(100, 100, BufferedImage.TYPE_IN\T_RGB);
        tree.paint(image.getGraphics());

        if (!Color.RED.equals(dRenderer.getBackgroundNonSelectionColor())) {
            throw new RuntimeException("Unexpected color, got: " +
                                       dRenderer.getBackgroundNonSelectionColor\());
        }
    }
}

with the following xml file:
<synth>
  <style id="backingStyle">
    <!-- Make all the widgets that use this skin opaque-->
    <opaque value="TRUE"/>
    <font name="Dialog" size="12"/>
    <state>
      <!-- Provide default colors -->
      <color value="YELLOW" type="BACKGROUND"/>
      <color value="GREEN" type="FOREGROUND"/>
      <color value="YELLOW" type="TEXT_BACKGROUND"/>
      <color value="YELLOW" type="TEXT_FOREGROUND"/>
    </state>
  </style>
  <bind style="backingStyle" type="region" key=".*"/>

  <style id="treeCellStyle">
    <opaque value="TRUE"/>
    <state>
      <color value="WHITE" type="TEXT_FOREGROUND"/>
      <color value="RED" type="TEXT_BACKGROUND"/>
    </state>
    <state value="SELECTED">
      <color value="RED" type="TEXT_FOREGROUND"/>
      <color value="WHITE" type="BACKGROUND"/>
    </state>
  </style>
  <bind style="treeCellStyle" type="region" key="TreeCell"/>
</synth>

Notice that null is returned from the renderer, the expected color is red.
###@###.### 10/21/04 23:18 GMT