Hi Sebastian.
Sorry, I didn't see your comment before.
Have you tried using data binding to change the color during runtime?
Another approach would be to build the toolbar in the code-behind using C# and then setting different colors based on some conditions.
As I am not near my computer at the moment, I won't be able to give a specific answer at the moment.
If you can't figure it out, let me know and I might write up another blog post about configuring/updating the toolbar from the code-behind during runtime.
Cheers
Julian