The links to the post include count of the number of replies. This means that when a new reply is made the count goes up one and the link changes. That's how your browser knows whether it's a new, unseen link or not. Problem is, when you make a reply, that also changes the count.
This is a browser thing, not a forum thing. Try this:
1. Click on the Subject of the post. when the post opens look at the URL in your browsers address bar.
2. Click on the Subject Date of the post. Again look at the URL in the address bar. They will be different. The Date post URL will end with a "#" followed by some letters or numbers. This tells your browser that you want to both look at a web page, and scroll down the page to a particular location in the page.
Since these two links are different, if you click on the Date, the Browser doesn't mark the Subject link as already seen by changing its color.
Well that's all very good, but why can't the Forum know that I've accessed a thread by clicking either way, and still changing the colour of the subject?
Isn't all it needs is to have two different triggers to changing the text colour?
After all, it isn't any other forum I've clicked on, it's this one!
The text color change is a built-in feature of your web Browser. If the web page designer chooses, this can be disabled on a per link basis, but there's nothing that says make a link appear as though it was previously visited. This would have to be done in programming code, and would require extra effort for very little payback.