private Ball getColorBallAt(Color color, Coords coords) {
      Integer col = coords.getCol();
      Integer row = Math.abs(coords.getRow());
      if (col < 0 || row > 4 || row + col > 8) {
         return null;
      }
      Ball ball = new Ball(color, coords);
      if (this.balls.contains(ball)) {
         return ball;
      }
      return null;
   }