你必须在CSS文件中使用选择器:nth-child(n)。Here's a link更好的解释。

.gallery-image:nth-child(1) {
  background: red;
} 

.gallery-image:nth-child(2) {
  background: green;
}

.gallery-image:nth-child(3) {
  background: blue;
}

如果这不是你要找的，告诉我。我很乐意帮忙

当你点击一张照片，它在一个弹出式模式中打开时，模式的背景每次都会不同。单击向左或向右箭头时，背景颜色将再次更改。
在你的JS代码中我添加了这一行

randomBackgroundColor(); //<-- NEW LINE

函数randomBackgroundColor()为popup元素的背景设置不同的颜色。
我希望我能帮上忙

示例：

const images = [...document.querySelectorAll('.image')];

const popup = document.querySelector('.popup');
const closeBtn = document.querySelector('.close-btn');
const imageName = document.querySelector('.image-name');
const largeImage = document.querySelector('.large-image');
const imageIndex = document.querySelector('.index');
const leftArrow = document.querySelector('.left-arrow');
const rightArrow = document.querySelector('.right-arrow');

let index = 0; // will track our current image;

images.forEach((item, i) => {
    item.addEventListener('click', () => {
        updateImage(i);
        popup.classList.toggle('active');

        randomBackgroundColor(); //<-- NEW LINE
    })
})

const updateImage = (i) => {
    let path = `img/img${i + 1}.png`;
    largeImage.src = path;
    imageName.innerHTML = path;
    imageIndex.innerHTML = `0${i + 1}`;
    index = i;
}

closeBtn.addEventListener('click', () => {
    popup.classList.toggle('active');
})

leftArrow.addEventListener('click', () => {
    if (index > 0) {
        updateImage(index - 1);
        randomBackgroundColor(); //<-- NEW LINE
    }
})

rightArrow.addEventListener('click', () => {
    if (index < images.length - 1) {
        updateImage(index + 1);
        randomBackgroundColor(); //<-- NEW LINE
    }
})

const setPopupImage = index => {
    popup.classList.toggle('active');
}

setPopupImage(0);


function randomBackgroundColor() {
    let red = Math.floor(Math.random() * 256);
    let green = Math.floor(Math.random() * 256);
    let blue = Math.floor(Math.random() * 256);

    let color = 'rgb(' + red + ', ' + green + ', ' + blue + ')';

    popup.style.backgroundColor = color;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

*:focus {
    outline: none;
}

body {
    width: 100vw;
    height: 100vh;
    display: flex;
    justify-content: center;
    align-items: center;
    background: #ff7a2d;
    font-family: 'roboto', sans-serif;
}

.gallery {
    width: 80%;
    height: 90vh;
    max-width: 1600px;
    max-height: 800px;
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    align-items: center;
}

.gallery-image {
    width: 30%;
    height: calc(50% - 20px);
    min-width: 300px;
    min-height: 200px;
    margin: 10px;
    overflow: hidden;
}

.image {
    width: 100%;
    height: 100%;
    object-fit: cover;
    transition: 1s;
}


.popup {
    position: fixed;
    top: 50%;
    left: 50%;
    transform: translate(-50%, -50%) scale(0);
    width: 100vw;
    height: 100vh;
    background: pink;
    display: flex;
    justify-content: center;
    align-items: center;
    z-index: 5;
    overflow: hidden;
    transition: 1s;
    opacity: 0;
}

.bk {
    width: 100vw;
    height: 100vh;
    background: brown;
}

.popup.active {
    transform: translate(-50%, -50%) scale(1);
    opacity: 1;
}

.popup.active .close-btn,
.popup.active .image-name,
.popup.active .index,
.popup.active .large-image,
.popup.active .arrow-btn {
    opacity: 1;
    transition: opacity .5s;
    transition-delay: 1s;
}

.top-bar {
    position: absolute;
    top: 0;
    left: 0;
    width: 100%;
    height: 10px;
    background: turquoise;
    color: #fff;
    text-align: center;
    line-height: 50px;
    font-weight: 300;
}

.image-name {
    opacity: 0;
}

.close {
    position: absolute;
    top: 35px;
    right: 70px;
    cursor: pointer;
}

.arrow-btn {
    opacity: 0;
    position: absolute;
    top: 50%;
    transform: translateY(-50%);
    padding: 10px;
    border-radius: 50%;
    border: none;
    background: none;
    cursor: pointer;
}

.left-arrow {
    left: 10px;
}

.right-arrow {
    right: 10px;
    transform: translateY(-50%) rotate(180deg);
}

.arrow-btn:hover {
    background: rgba(0, 0, 0, 0.5);
}

.index {
    position: absolute;
    bottom: 10px;
    right: 10px;
    font-size: 80px;
    font-weight: 100;
    color: rgba(255, 255, 255, 0.4);
    opacity: 0;
}

.large-image {
    width: 90%;
    height: 90%;
    object-fit: contain;
    opacity: 0;
}

<div class="popup">

    <div class="top-bar">
        <p class="image-name">img1.png</p>
        <span class="close-btn">
            <img src="Graphics/gallery_icon.svg" width="50px" class="close">
        </span>
    </div>

    <button class="arrow-btn left-arrow"><img src="img/arrow.png" alt=""></button>
    <button class="arrow-btn right-arrow"><img src="img/arrow.png" alt=""></button>

    <img src="img/img1.png" class="large-image" alt="">

    <h1 class="index">01</h1>
</div>

<div class="gallery">
    <div class="gallery-image">
        <img src="img/img1.png" alt="" class="image">
    </div>
    <div class="gallery-image">
        <img src="img/img2.png" alt="" class="image">
        <div class="bk"></div>
    </div>
    <div class="gallery-image">
        <img src="img/img3.png" alt="" class="image">
    </div>
    <div class="gallery-image">
        <img src="img/img4.png" alt="" class="image">
    </div>
    <div class="gallery-image">
        <img src="img/img5.png" alt="" class="image">
    </div>
    <div class="gallery-image">
        <img src="img/img6.png" alt="" class="image">
    </div>
</div>