void setup(){
  size (400,400);
  stroke(250,250,250);
}

int value = 0;

void draw () {
  fill(value);
  rect(400, 400, 400, 400);
} 

void mouseDragged () {
  line(0,0,mouseX,mouseY);
  line(0,400,mouseX,mouseY);
  line(400,0,mouseX,mouseY);
  line(400,400,mouseX,mouseY);
  }

void mouseMoved ()

{

  value = value + 5;
  if (value > 255) {
    value = 0;
  }
  
}