Green is generally used as symbolism for hope, change, rebirth, which are themes that obviously apply to this story, not to mention go along with the swampy environment. Washing out the scenes with the same hue also adds to the harsh feel of the life they were trying to portray. They talk a little about it here: https://www.distractify.com/p/is-emancipation-movie-in-black-and-white
I guess I'd rather have them fill the screen with those colors organically rather than alter it after the fact. The final product was just too dull to look at for me. For comparison, the movie Rescue Dawn comes to mind. Story of a Vietnam pilot escaping prison and being pursued while battling the jungle elements. They filled the scenes with green.