In long lists of links, it's helpful to show users which links they've already followed so they can focus on unseen content. Browsers do not tend to let you style visited links with anything but the color property.
Examples
✓ Good example
With link styling, as with many aspects of web-interface design, you can aid comprehension by capitalizing on convention. By default, browsers style unvisited links with a #0000EE blue and visited links with a #551A8B purple. These colors will be the easiest identifiers at your disposal.
✗ Bad example
It is possible to assign a special color to visited text using the :visited pseudo-class in CSS. However, this breaks with convention, so it is not recommended. Different colors may look "nicer," but breaking with convention diminishes comprehension and usability.
As a content author, avoid applying custom colors to links using the Select Text Color tool. All links should have the same appearance so that users can easily identify them. In addition, applying a link color manually is likely to override and suppress the visited style of the link.