void setup(){
  size(250,250);
  noStroke();
  rectMode(CORNERS);
  background(0,0,255);
}
void draw(){
  for(int i=0;i<=120 && i< mouseX;i+=10){
      fill(i*2,0,250-i*2);
      rect(i,i,250-i,250-i);
  }
}