Thanks for that, Phil. That's what I thought.
Some of those texts are actually projects. So as a test I added
\Marker p
\Color 16711680
in one of the custom.sty files.
After that the main text of that specific project is indeed displayed in blue when I open it separately as a project and also on the right side in a text collection.
However, the text color is still black on the left side of the text collection. Thus, it doesn't do what I had in mind. I was hoping that when I scroll through the different versions in the text collection, the color would show me quickly that I'm now in the section of that specific language. (I have sorted the versions in the text collection according to their language.)
Anyway, I don't think it's a big deal.
Thanks again!