public void fillRandomly() {
      for (int i = 0; i < rows; i++)
         for (int j = 0; j < columns; j++) {
            int r = (int)(256*Math.random());
            int g = (int)(256*Math.random());
            int b = (int)(256*Math.random());
            grid[i][j] = new Color(r,g,b);
         }
      forceRedraw();
   }