Oil Paint

The back wall, and the flagstones on the floor were done in one painting session.  I also changed my mind about the shape of her couch.
Oil  paint is heavy and dense and you can cover up previous marks very easily if you want to, or thin it down and make transparent glazes if you want to.
All of this went down quickly and naturally.   It's background and it doesn't need to be super detailed or pretty or brightly colored in fact it shouldn't be because it would detract from the characters.

There may be several points in the process of a painting where you begin to doubt yourself.  It doesn't look like you were hoping, it may seem to be falling apart.  This is normal.  One teacher calls it the "ugly phase" and he says that every painting has them.  All you you can do is work through it, and know that it will get better.
 It will.   ...or maybe it won't.
Another teacher was fond of saying:"You know how to do one good painting?  Do ninety nine bad ones." 

 Hyperbole one hopes, but there's a lot of truth in it.