Version:           3.0.3 (using KDE KDE 3.0.5)
Installed from:    SuSE RPMs

I would like to mark long periods of time using colors or colored lines. Examples are: Marking days as Semester or break, as school day or school holiday. Mark for a period of belonging to a certain project, of period of employment at a certain employer, etc.

It is important that multiple such marks can be active at any time.
Therefroe fimply changing the background color would not be sufficient. It would be better to make vertical or horizontal lines (depending on the view) of different colors.
The lines should probbaly not have text that is usually displayed, maybe only a tooltip.