正如评论者所说，它是通过mix-blend-mode css属性完成的。Official documentation。

这里有一个概念验证：

div.circle {
  display: inline-block;
  width: 30px;
  height: 30px;
  border-radius: 15px;
  background: green;
}

h1.header {
  display: inline-block;
  color: darkgray;
}

h1.logo {
  position: absolute;
  top: 0;
  color: lightgray;
  mix-blend-mode: difference;
}

<div class="container">
  <div class="circle"></div>
  <h1 class="header">Welcome to StackOverflow!</h1>
  <h1 class="logo">Hello</h1>
</div>