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.
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."