The text color is lighter, as if the post had been downvoted.
On my computer, the default color for visited links is also lighter.
That's the 'visited link' color, which indicates that you've visited a given post's link. Posts do not change color based on votes. For posts without a link, reading the post requires still visiting the post's link, so it works out as expected from there.
No, the actual colour of the text on text-only posts is super light. I read somewhere a while back that it was supposed to discourage people from using it too much.