int x = 0;
float a=random(0,255);
float b=random(0,255);
float c=random(0,255);

void setup() {
  size(600,600);
  background(0);
  strokeWeight(3);
  frameRate(10);
}

void draw() {
  background(0);
  stroke(a,b,c);
  fill(0);
  rect(200+x,200+x,200-x,200-x);
  a=random(0,255);
  b=random(0,255);
  c=random(0,255);
  if(mouseY>300){
    x=x+3;
  }if(mouseY<300){
    x=x-3;
  }
}