Computer Help and Support

Normally, you open a link and it is recorded in your browsing history. That's how the browser knows that you opened it, and changes the color.

Best guess is that the history is getting cleared in a browser setting (this is not the usual default) when you close the tab, (or quit and restart the browser) and when you go back, it's not in your history and is the normal "unread" color.

Cruise through preferences, probably under "privacy".

HTH


Anecdotally:
When I clear everything manually (history, data, cookies) I have to log in to DU again (it's the cookies that do this). But realize that there are three separate items we are talking about, and you can change how they are kept or cleared. Looks like you got a default that doesn't suit you.