Why is there still an Air Berlin branded plane operating as of Sep 5th, 2018?
Paintshops have been very busy in the past year.
So getting a slot to repaint an airplane is not that easy. 
In addition, apart from the fact that the livery is not the one of the owner, the plane is perfectly fine. 
And while applying a new paint scheme the airplane is not earning money. 

What airlines do is, repainting the planes when a longer technical check is due anyways.