关于after或before伪类选择器透明度实现问题

after伪类选择器 background-color:rgba(0,0,0.5); 样式实现为纯黑，（透明度不正确）
<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        *{
            margin: 0;
            padding: 0;
        }

        #rollpic1{
            width:2732px;
            height: 840px;
            
            margin: 50px auto;
            font-size: 0px;
            overflow: hidden;
            position: relative;
        }

        #select1{
            width: 400px;
            height: 100%;
            position: absolute;
            /* background-color: red; */
            left: 400px;
            top: 0;
        }
        #select1 ul{
            width: 400px;
            height: 100%;
        }
        #select1 ul li{
            width: 100%;
            height: 16%;
            background-color: rgba(0, 0, 0, 0.7);
            margin: 2px 1px;
            font-size: 22px;
        }
        #select1 ul li p{
            color: white;
            left: 20%;
            width: 80%;
            height: 40%;
            top: 20%;
            position: relative;
        }


        .ul2li::after{
            content: "";
           position: absolute;
           top: 0;
           left: 410px;
           width: 600px;
           height: 100%; 
           background-color:rgba(0,0,0.5);
           /* 透明度不正确 */
        }

    </style>
</head>
<body>
    <div id="rollpic1">


        <div id="select1">
            <ul>
                <li id="li201" class="ul2li"><p>1</p></li>
                <li id="li202" class="ul2li"><p>2</p></li>
                <li id="li203" class="ul2li"><p>3</p></li>
                <li id="li204" class="ul2li"><p>4</p></li>
                <li id="li205" class="ul2li"><p>5</p></li>
                <li id="li206" class="ul2li"><p>6</p></li>
            </ul>
        </div>

    
    
    
</body>
</html>
你少写了个0 啊 background-color:rgba(0,0,0,0.5);