If I were doing my own impression of the paining and not a copy, I wouldn’t have painted the background that way. I would have textured it in some way. Maybe with shadows, a variation of a light side and a dark side, just to break up the flatness of the background. He has no shadows to speak of. You can manipulate shadows to draw interest where there is no interest.
Doing an impression and doing a copy are different because, for the latter, I’m trying to represent another painter’s choices. At the National Gallery, I’m a copyist, not an improv.