This problem is even more apparent if you look at the colours used in the list of questions itself.
Here are two questions on ELU:
I've already visited "Irreponsible person" but I've yet to visit "why the use' much of which ' in the following sentence is incorrect?"
My normal routine is to copy the title of the question I know I've last visited, refresh the page, and then search for the title of that question. Either that or try to remember its title myself.
The differentiation between the two colours is so minimal, I doubt I'd ever spot it based on colour alone.
On the other hand, compare the difference in colours used at ELL:
On ELL, I can spot the difference and not worry about losing track of what questions I have and haven't already looked at.
I'm not certain if this needs to be broken down into visited versus non-visited link-text colours in different areas, but I'm hoping a single feature request can cover a review of the colours used in all areas.
Update: I finally got so tired of this, I implemented a workaround of my own.
I installed the Firefox extension Stylus. (It is also available for Chrome.)
I then created the following style:
In text, that is:
color: grey !important
URLs matching the regexp: https://.*.stackexchange.com/questions
URLs matching the regexp: https://.*.stackexchange.com/questions\?sort=newest&page=\d*
Now, on those Stack Exchange pages that list questions, every link to the questions I've visited are in grey text. I no longer have a problem knowing where to start looking when I review new questions.