<style>
    div{
        width:300px;
        height:200px;
        background-color: pink;
    }
</style>
<script>
    //隨機產生一個十六進位制的值
    function getColor() {
        var str="#";
        //一個十六進位制的值得陣列
        var arr=["0","1","2","3","4","5","6","7","8","9","a","b","c","d","e","f"];
        for (var i=0;i<6;i++){
            //產生的每個隨機數都是一個索引，根據索引找到陣列中對應的值，拼接到一起
            var num=parseInt(Math.random()*16);
            str+=arr[num];
        }
        return str;
    }
    window.onload=function() {
        document.getElementById("dv").style.backgroundColor = getColor();
    };
    
</script>