为什么css红绿蓝三种颜色叠加不是黑色呢？

效果
测试代码

<!DOCTYPE html>
<html>
<head>
    <title></title>
</head>
<body>

<div class="x">
<div class="a item"></div>
<div class="b item"></div>
<div class="c item"></div>
</div>

</body>
<style>
.x{
    width: 500px;
    height: 500px;
    /*background-color: red;*/
    margin: 200px;
    border:1px solid black;
    position: relative;
}
.item{
    width: 100px;
    height: 100px;
    opacity: 0.33333;

}
.a{
    background-color: red;
}
.b{
    background-color: green;
    margin-top:-20px;
}
.c{
    position: absolute;
    left: 80px;
    top:0;
    background-color: blue;
}
</style>
</html>

前端 css css3 html

html5