Detecting a recolored area of ​​the screen in java

I am creating an applet that will do something like screen sharing. To do this, I use the Robot class to capture an area of ​​the screen. I want to capture the dirty / recolored areas after the first screenshot. I think the RepaintManager can be used for this. but i dont know how i can use it. I am a beginner so the Java code will be helpful. thank