float r1;
float r2;
float r3;
float r4;

void setup(){
  size(200,200);
}

void draw(){

  stroke(0);

  fill(r1);
  rect(0,0,100,100);
  if(mouseX <= 100 && mouseY <= 100){
  r1 = 0;
  }else{
  r1 = 255;
  }

  fill(r2);
  rect(0,100,100,100);
  if(mouseX <= 100 && mouseY > 100){
  r2 = 0;
  }else{
  r2 = 255;
  }

  fill(r3);
  rect(100,0,100,100);
  if(mouseX > 100 && mouseY <= 100){
  r3 = 0;
  }else{
  r3 = 255;
  }

  fill(r4);
  rect(100,100,100,100);
  if(mouseX > 100 && mouseY > 100){
  r4 = 0;
  }else{
  r4 = 255;
  }

}