你可以用 document.querySelectorAll 获取所有锚元素并在其上循环以设置颜色(注意 document.querySelectorAll 优先于 document.getElementsByClassName 因为返回的集合不是活动的。）

const nav = document.getElementById('centered_nav');
const logo = document.getElementById('rc_logo');
const links = document.querySelectorAll('.link1');

window.onscroll = function(){

    if (window.pageYOffset > 100) {

        nav.style.background = "#FFFFFF";
        nav.style.boxShadow = "0px 2px 1px #E55200";
        links.forEach(link => link.style.color = "#000000");
        logo.style.backgroundImage = "url('http://localhost/assets/img/nav-logo-color.png')";
    }
    else {
        nav.style.background = "transparent";
        nav.style.boxShadow = "none";
        links.forEach(link => link.style.color = "#FFFFFF");
        logo.style.backgroundImage = "url('http://localhost/assets/img/nav-logo.png')";
    }
}

直接的答案是您影响了许多链接，而不是一个链接，因此您需要创建一个集合并使用类似的方法对其进行迭代 forEach . 在您使用的代码中 document.getElementsByClassName('link1')[0]; 这只是第一个环节。

const nav = document.getElementById('centered_nav');
const logo = document.getElementById('rc_logo');
const links = document.getElementsByClassName('link1');

window.onscroll = function(){
    if (window.pageYOffset > 100) {
        nav.style.background = "#FFFFFF";
        nav.style.boxShadow = "0px 2px 1px #E55200";
        links.forEach(link => link.style.color = "#000000" );
        logo.style.backgroundImage = "url('http://localhost/assets/img/nav-logo-color.png')";
    }
    else {
        nav.style.background = "transparent";
        nav.style.boxShadow = "none";
        links.forEach(link => link.style.color = "#ffffff" );
        logo.style.backgroundImage = "url('http://localhost/assets/img/nav-logo.png')";
    }
}

更好的答案是只设置一个类 body 也许，然后让您的css规则完成其余的工作。你甚至可以通过这种方式添加过渡，这将允许你设置从白色到黑色等过渡的动画。我把过渡规则放在这里，你可以根据需要删除。

const body = document.querySelector('body');
window.onscroll = function() {
  if (window.pageYOffset > 100) body.classList.add('scrolled')
  else body.classList.remove('scrolled')
}

css：

# rc_logo {

  background-image: url('http://localhost/assets/img/nav-logo.png');
}

# centered_nav {

  background: transparent;
  box-shadow: none;
  transition: background .5s linear .5s;
}
.link1 {
  color: #fff;
  transition: background .5s linear .5s;
}

body.scrolled #rc_logo {
  background-image: url('http://localhost/assets/img/nav-logo-color.png');
}
body.scrolled #centered_nav {
  background: #000;
  box-shadow: 0px 2px 1px #E55200;
  transition: background .5s linear .5s;
}
body.scrolled .link1 {
  color: #000;
}