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How to make a JFrame one colour?

I'm trying to add some polish to a small game I made and, every time a round finishes, I'd like the whole JFrame to flash white and quickly lose opacity until you can see the game again. How would I do this?
Note: I'm thinking about hiding all the components and adding a JPanel on top that loses opacity quickly but this doesn't seem right (and I'd like to be able to see the components behind the white flash as it goes from opaque to transparent).

You could probably use the JLayer class. Take a look at the section from the Swing tutorial on How to Decorate Components with the JLayer Class.
The tutorial has examples that:
paint translucent layers
do animation
Put the examples together and you should have a solution.

You can use fading out image to achieve the same. AlphaComposite is used for fading effect.
Graphics2D g2d = (Graphics2D) g;
g2d.setComposite(AlphaComposite.getInstance(AlphaComposite.SRC_OVER, alpha));
Refer Fade Out Image to access the sample code.

Trouble with Java Graphics2D

In my game, I'm using different kinds of objects that implement the Paint interface including GradientPaint for the background. When I try to paint the background, it mostly leaves a grey border on the right and bottom sides of the screen; I don't want that border there. How can I get rid of it?
I notice that when I set my JFrame to be resizable instead of not, the background paints okay as want, but then people will be able to resize the window and distort the visuals needed to play the game.
What I am using to draw graphics is the outdated AWT, using both Graphics and Graphics2D. All the backgrounds I'm painting consist of either one, two, or four Rectangle2D.Float objects, with the appropriate sizes. For a full background, it would be 0, 0, gamePanel.getPreferredSize().width and gamePanel.getPreferredSize().height.

Faulty repaint() of swing components when using custom background colors

When I use JCheckboxes or JScrollPane (applied to the main component that holds all others in order to generate a scrollable window) together with components that use
component.setBackground(new Color(R, G, B, A));
to define their background color, I am getting some obnoxious repaint() issues. Hovering over JCheckboxes activates MouseListener and the background of the JCheckbox will suddenly display a random other part of the window. This remains even when taking the mouse off the JCheckbox.
The issue disappears when using
JCheckbox.setRollOverEnabled(false);
BUT will still occur when selecting the checkbox!
The scrollpane will also not properly repaint. ONLY the parts that are outside of the visible frame will be painted several times in a row in direction of scrolling when they come back into the frame. It looks similar to that error on Windows OS when a program crashes and you can "draw" with the window on the screen because it "generates" a new window every time you move it (http://i.stack.imgur.com/L5G5Q.png).
The most interesting part is that the issue completely disappears when I use
Color.grey (or any other pre-generated color)
It also disappears when not selecting a custom background color at all.
So is there an issue with revalidate() and repaint() hidden anywhere in this? Is the use of RGBA a problem, specifically the A (= opacity) part since Color.AnyColor works?

Is the use of RGBA a problem, specifically the A (= opacity) part
Yes, Swing does not support transparent backgrounds.
Swing expects a component to be either:
opaque - which implies the component will repaint the entire background with an opaque color first before doing custom painting, or
fully transparent - in which case Swing will first paint the background of the first opaque parent component before doing custom painting.
The setOpaque(...) method is used to control the opaque property of a component.
In either case this makes sure any painting artifacts are removed and custom painting can be done properly.
If you want to use tranparency, then you need to do custom painting yourself to make sure the background is cleared.
The custom painting for the panel would be:
JPanel panel = new JPanel()
{
protected void paintComponent(Graphics g)
{
g.setColor( getBackground() );
g.fillRect(0, 0, getWidth(), getHeight());
super.paintComponent(g);
}
};
panel.setOpaque(false); // background of parent will be painted first
Similar code would be required for every component that uses transparency.
Or, you can check out Background With Transparency for custom class that can be used on any component that will do the above work for you.

How to solve graphical glitching with JDialog, JPanel?

So the intial setup for this issue is that there's a JDialog, and inside that I've placed a JPanel that would house the rest of the components (since painting the JDialog itself is apparently a bad idea). This JPanel has an overriden paintComponent(Graphics g) method that only paints the background and adds a faint border for aesthetic purposes.
Now inside that is a series of JPanels that categorize the contained form components, and each JPanel has an overridden paintComponent(Graphics g) as well, painting a semi-transparent background.
Inside each of those JPanels is where I start to have some issues, presumably with transparency. I have JTextFields, JCheckBoxes, JLabels, JSliders, etc inside these panels, and when you interact with one (hover, click, etc), the background goes from transparent to opaque, with an occassional ghosted image from another field (which appears slightly random sometimes). I'm using a custom LAF called Web, but I tested with other built-in LAFs and the same thing happens.
Is this a glitch with Java or did I mess something up? If so, how can I patch this up? I can paste code fragments later if necessary, but I've used several custom classes and nine-patch style image stitching which may make the code fragments hard to follow. Thanks in advance!

If you are painting components with a transparent background it is very important that the component is marked as transparent (setOpaue(false)) so that the repaint manager knows that it must paint the components below it.
It is also very important that when you are performing custom painting that you call super.paintComponent first.
This is especially important in the case of transparent components, as this prepares the Graphics context for painting.
Graphics is a shared resource. All the components painted in your window will share the same Graphics object, meaning that if you don't allow paintComponent to first prepare it, then you will see what was previously painted on it.

Fading a portion of Graphics in Java Swing

I am trying to implement the fade in/fade out animation in swing.
I am using a JPanel which does not have any components in it. It is completely drawn by paintComponent() method.
Now in one of the portion of this JPanel, I want to implement the fade in/fade-out animation. When I tried using AlphaComposite, the animation is being triggered for whole JPanel.
Can I limit this animation in a small clipped region in that panel?
Graphics2D g2d = (Graphics2D) g;
g2d.setComposite(AlphaComposite.getInstance(
AlphaComposite.XOR, alpha));

Have you tried using an Graphics object (like rectangle, circle etc..) for your fade in/out? That way it won't be triggered for the complete panel.
Good luck!

Perhaps, but that may be more difficult to achieve than what it's worth. Create a JComponent of the size you want to animate (or fade), add it to your JPanel, and have repaint() called on your smaller component during animation instead of the larger JPanel.

You can use setClip() before painting to restrict the fading area.
Suppose you want a small fading rectangle. Using Area class create 2 Shapes. Intersection of original clip and fading rect and subtraction (subtract the fading rectangle from the original clip).
Call super.paintComponent() twice with 2 different clips. For the second paint you can set your alpha filter.