Just invoking method update() on the SplashScreen instance changes the color of the involved pixels. I.e. if we show the splash screen, take a snapshot of the central pixel, invoke createGraphics() and update(), take a snapshot again - the colors will differ.
The following code illustrates the issue (complete source is attached):

    private static void test() {
        SplashScreen ss = SplashScreen.getSplashScreen();

        sleep(250);
        Color initialColor = getCentralPixelColor();
        Graphics2D graphics = ss.createGraphics(); // avoid ISE
        ss.update(); // if commented out - the test passes
        sleep(250);
        Color updatedColor = getCentralPixelColor();

        if ( ! updatedColor.equals(initialColor)) {
            System.out.println("The color of the central pixel changed " +
                               "after invoking update()!");
            System.out.println("Initial color: " + initialColor);
            System.out.println("Updated color: " + updatedColor);
        }
    }

Sample execution on Ubuntu 10.04 (24-bit color):

$ java -splash:splash-white-80x80.png Test
The color of the central pixel changed after invoking update()!
Initial color: java.awt.Color[r=255,g=255,b=255]
Updated color: java.awt.Color[r=254,g=254,b=254]