repaint in a loop

You can use JComponent.paintImmediately to force an immediate repaint.

EDIT: After reading your question again it occurs to me that you might be executing your logic on the event dispatch thread. This would mean that the repaint requests will not be executed until after your method returns. If you put your code into another thread then that will probably fix the problem and it will be a lot nicer than using paintImmediately.

void uiFunction() {
    new Thread() {
        public void run() {
            for(int i = 0; i < 10; i++) {
                final JButton b = buttons[i];
                SwingUtilities.invokeLater(new Runnable() {
                    b.setBackground(Color.WHITE);
                    b.repaint();
                }
                Thread.sleep(2000);
                SwingUtilities.invokeLater(new Runnable() {
                    b.setBackground(Color.GRAY);
                    b.repaint();
                }
            }
        }
    }.run();
}

It's a bit mucky but hopefully it gives you an idea of where to begin.

Not sure if I'm on the right track here, but is this a thread issue? Because the sleep will be putting the current thread to sleep. So if the repaint is on the current thread, you will be effectively queuing a repaint, holding, then queuing another repaint. Then the method ends and the event processing loop of the graphics thread fires, executing two repaints (collapsed) making everything happen at once.

Based on that thought I think you might have two options:

1. Put the code on a second thread so that it doesn't halt the main graphics thread and also because it means that the graphics thread cando a repaint along side the sleep happening.

2. Before you sleep (not really recommended I would think), trigger the event loop to force the repaint to happen. I'd have to look this up. Cannot remember how to do it off the top of my head. :-)

I'd probably be looking at #1 as the best option.