I had to scroll back for a while but I found a topic that was bumped, and I had read all posts back then.
Here is a screenshot. You can see the color is --primary just like all the other topic titles.
You can also see that the link has /34 at the end, which is the “automatically bumped on Aug 11” post.
So how should I have recognized this as a bumped topic?
When I press the back button, then the title becomes grey. Because I then indeed saw every post in that topic, including the “automatically bumped” post, the only post I hadn’t read.
So it seems like even the “automatically bumped” post itself prevents the title from being grey.