Hello everyone,
I've solved this problem with a bit of tricky programming, so in case anyone
else is wondering how to do this I'm going to post the solution here.  It's
kind of a kludgy hack but it works.

Normally, to paint the icon, you would use this:
icon.paintIcon(c, g, x, y);

where c is the component, g is the Graphics2D context, etc.

To paint the highlighted icon, use this instead:

BufferedImage tmpImage = new BufferedImage(icon.getIconWidth(),
icon.getIconHeight(), BufferedImage.TYPE_INT_RGB);
Graphics2D tmpGfx = tmpImage.createGraphics();
icon.paintIcon(c, tmpGfx, 0, 0);    //lay down the original icon
tmpGfx.setXORMode(new Color(0, 0, 80));     //replace this color with your
highlight color of choice

//this will make an area of solid highlight color wherever the icon was
painted (but not where it was transparent)
icon.paintIcon(c, tmpGfx, 0, 0);

tmpGfx.dispose();    //you must free up native resources

icon.paintIcon(c, g, x, y);    //lay down original icon in our main gfx
context.

ImageIcon highlight = new ImageIcon(tmpImage);
Composite tempCmp = g.getComposite;    //save the current composite
g.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, 0.5f));
/*set the alpha blending, you can fiddle with the alpha value if you like*/
highlight.paintIcon(c, g, x, y);    //overlay the highlight
g.setComposite(tempCmp);    //restore the original composite.

That's it.

-Jeremy