一个相当简单的解决方案是创建一个背景颜色数组，其数量与图像的数量相匹配：

const backgroundColors = [
  '#fff',
  '#b2fa27',
  '#c2ba27',
  '#d27aee',
  '#e27a27',
  '#000',
];

然后，基于HTML结构，您可以创建一个函数来更新每个图像的背景颜色：

const updateBackground = (i) => {
    popup.style.backgroundColor = backgroundColors[i];
}

最后，你可以在初始化弹出窗口和点击事件时调用这个函数：

leftArrow.addEventListener('click', () => {
    if(index > 0){
        updateImage(index - 1);
        updateBackground(index);
    }
})

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

const setPopupImage = index => {
  popup.classList.toggle('active');
  updateBackground(0);
}

下面是一个完整的示例，演示了上面所做的更改：

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

// popup

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');

const backgroundColors = [
    '#fff',
  '#b2fa27',
  '#c2ba27',
  '#d27aee',
  '#e27a27',
  '#000',
]

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

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

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

const updateBackground = (i) => {
    popup.style.backgroundColor = backgroundColors[i];
}

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

leftArrow.addEventListener('click', () => {
    if(index > 0){
        updateImage(index - 1);
        updateBackground(index);
    }
})

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


const setPopupImage = index => {
  
  popup.classList.toggle('active');
  updateBackground(index);
}
setPopupImage(0);

* {
  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;
}



.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;
}

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Image Popup</title>

    <link rel="stylesheet" href="style.css">

</head>
<body>
    <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>

    <script src="index.js"></script>
    
</body>
</html>