void setup() {
  size (200,200);
  stroke(255);
  background(0); // black
  frameRate(60);
}

int step = 5;
float y = step;
float x = step*2;
int yDir = 1;
int xDir = 1;
int lineColor=0;
float rr=0;
float rb=0;

void draw() {
  //

 
  //if (y >= (height-step) || y <= 0 ) { yDir *= -1;  }
  //if (x >= (width-step) || x <= 0 ) { xDir *= -1; }
  if (y > (height-step) || y<=0) { yDir*= -1; xDir*=-1;}
  //if (x > (width-step)) { x=step; }

  y += (yDir * step);
  x += (xDir * step);
  
  
  stroke(rr,20,lineColor);

  if (lineColor < 255) {
    lineColor++;
  } else {
    if (lineColor == 255) { 
      rr=random(255);
      rb=random(255);
      lineColor=0; 
      //background(0); 
    }
    lineColor--;
  }
  
  if (y == step && x == step) {
    background(0);
  }
  
  line(0+2, x, y, height-2);
}