This is kind of random, but I was just writing an email about it so I figured I’d document it here:
There are certain instances in which visited links changing color are useful, but often they are not and degrade the coherence of the page.
We probably all know visited links changing color (blue to purple) was a strong/default convention applied across all pages in all situations in the early days of the Web. Not so much anymore as designs have become more refined and people have become more experienced using the Web.
A more recent convention has been to have visited links (if they change at all) change to a faded version of the link color rather than a completely different color. NY Times does this on its article headlines. Apple doesn’t change visited link colors at all. Google search sticks to the old convention.
If a page is information-oriented (search, bunch of FAQs, archive, help, etc.) and there are an abundance of links that a user might visit and then return to the originating page, visited links are useful (Google search example) — they actually save the user time and trouble. If a page is a long list of articles, the same applies (NY Times example). In the case of a page that has just a few links there may be little or no advantage, though there may be no harm either. Otherwise, changing link color is a detriment to the User Experience — just about any time they are used in any kind of navigation menu, and any time the factors mentioned do not apply. If there are rare cases where they would be helpful and generally they would not be helpful, I would abandon them in the interest of overall consistency for the site.