int x,y,dx,dy;
float distance;

void setup(){
  size(300,300);
  noStroke();
}

void loop(){
  for(y = 0; y < 300; y += 10){
    for(x = 0; x < 300; x += 10){

      dx = mouseX - x;
      dy = mouseY - y;
      distance = sqrt(dx*dx + dy*dy);

      fill(255-distance*2,dx,dy);
      rect(x,y,10-distance/30.0,10-distance/20.0);
    }
  }
}