Javascript 滚动时视差效果不适用于背景色

我的视差效果有问题。我有一张照片，在这张照片的背面我有一种颜色。下面是我的html

<div class="rectangle background-medium-blue header-stage-medium-blue-rectangle"></div>


<div class="image">
    <img class="img" src="stage-allviewports-4x3.jpg" width="1450" height="1089" />
</div>

css

---

请发布CSS。@Passele我添加了CSS请发布CSS。@Passele我添加了CSS

var parallax= document.querySelector(".header-stage-medium-blue-rectangle");

window.addEventListener("scroll", function() {

    var scrolledHeight= window.pageYOffset;

    parallax.style.top=  (scrolledHeight * 0.2) + "px";
});

.header-stage-medium-blue-rectangle {
    position: absolute;
    background-color: red;
    right: 0px;
    top: 302px;
}