在1-255之间随机生成数，m-n之间随机生成数

1.封装一个随机函数

  function randomTime(max, min) {
    let r = Math.floor(Math.random() * (max - min + 1) + min);
    let g = Math.floor(Math.random() * (max - min + 1) + min);
    let b = Math.floor(Math.random() * (max - min + 1) + min);
    return `rgb(${(r, g, b)},${(r, g, b)},${(r, g, b)})`;
  }
  console.log(randomTime(1, 255));