Today, I stumbled upon https://parenting.stackexchange.com/ browsed coupled of threads and noticed that site is creating confusion to users [personal experience] using dull color for non visited link and dark color for visited. Is not it breaking any UX Rule. UX.SE doesn't highlight or do nothing with link, Stackoverflow however change visited link color
In a nutshell: all 3 sites are using different colors for visited and unvisited links, with a similar degree of contrast. This is a best practice in web design, but other factors are considered when choosing the exact color pallet.
edit: Apperantly you're not the first user to think the contrast isn't high enough: https://meta.stackexchange.com/questions/137438/increase-link-color-contrast-used-on-se-sites
The best practice in link design is that visited links should look different from unvisited links. This helps users orient themselves in the system, indicating whether they've been to a page without having to actually open it and examine its contents. For a broader coverage of this best practice, read: Change the Color of Visited Links by Jakob Nielsen.
The colors for visited and unvisited links are different on all the websites you've mentioned, including this one. You can use your browser's "Inspect Element" feature, or any color picker tool to ascertain this. The difference in color looks greater in StackOverflow because they use a bold font in their link titles, in which each character has more "ink", making it easier to distinguish the colors. But essentially, all of the website you've mentioned have a similar degree of color difference between visited and unvisited links.
Although the degree of color contrast seems negligible to you, it's important to remember the conveying whether a a link is visited or not is not the main purpose of these websites' design. Giving visited links a more contrasting color would draw more attention to this detail, which might be counterproductive. At the end of the day, this is an interplay of usability and information hierarchy. The fact that you've confused a visited link for an unvisited one might be indicative of a usability problem other users are having, but without thorough user testing it will be hard to determine it.
Changing link color is a very old web convention, and can be seen on many of the big sites that people know: Google, Yahoo, Reddit, etc.
a:visited is even standard HTML behaviour if no other style is applied through CSS.
While it'd be great if anyone has research on this, i'd be very surprised if users are being confused by this – as long as it follows convention, namely the visited link being only a slight (usually darker) variation of the normal color, and nothing crazy like changing from blue to green.
In the case of Parenting.SE, I think they slightly overdid it: Compared to the visited, the unvisited links indeed look too dull, and could make people think they're somehow inactive.
I compared the contrast between the Parenting.se and Google:
It seems to me the contrast that parenting uses is indeed a bit stark, and appears off – even more so if you look at the actual letters and not at the grey squares.