I've managed to generate the process Diagram image which depicts the current state of the process, i.e. the current task border is colored with red.
However I'd like to highlight the tasks the process traversed until the current state, Or At least annotate the tasks left behind with some text on my own.
What guidance do you need? I suggest you look into the History API (HistoricActivityInstance, to be specific). You can get a list of all completed activities for a specific process-instance. Use those to highlight the correct activity in the process definition…
So you advise me to take that way in the link I posted. Is it that? API doesn't suplly, by itself , any way to highlight different activities with different colores, and a way to highlight the sequence flows?
That's indeed the way to go to extend the current behaviour in the generator. There has been some refactoring in the generator, though, so it may require some additional research…