Even though the user deleted their account before getting an answer, this was a good question.
As far as my experiments went, they were correct: In “light” mode, the text on the active tab’s title follows the foreground colour setting exactly. In Dark Mode, the text on the active tab’s title is some “inverse” of that, but not the normal “inverse”, so it is very hard to tell what the final color of the tab’s text will be, based on that setting.
If there is someone who is interested in getting that fixed, you should go to the Feature Request FAQ and follow those instructions to put in an official feature request. That official feature request should contain a link to this discussion (so that the developers know that it’s not just you who want the fix), and you should come back and put a link to that feature request in this discussion (so that readers here can find the issue, and see whether it is fixed in a future version of notepad++).