The colors are just for style and a bit of usability. The color is determined by the first letter of the domain, for example here is the CSS for domains starting with 'w':

  .comhead>a[href^="from?site=w"] {
    color: #0098ce !important;
  }

There is also a extra rule for Apple domains (domains ending with 'apple.com') that adds an apple icon to the left of the domain:

  .comhead>a[href$="apple.com"]:before {
    content: "---";
    color: #ffffff00;
    background: var(--mac-apple-icon) 0px;
  }