visualizing JPanel change within a loop

Question

I am new to Java swing programming. I want to make a frame which will appear red and blue in turn one after another. So, I took 2 child JPanel, 1 for red and other for blue, and

Answer 1

I would suggest to take in one step at a time.
First make it run without changing panels / colors.
Now it doesn't because this

public final void Test2() {

is a method (which is never used) and not a constructor. Change to a constructor declaration like :

public  Test2() {

to make the program do something. Then you can go to the next step.

Also use Java naming conventions (like blue instead of Blue).