I was studying the effects of different colors. With this color choice I got pretty close to a "swamp world" feeling. (Also it reminds me of the classic GameBoy colors a little.)
I am playing a lot with color effects lately. As an example, when Gray is picked by the player, colors could drift to a specific configuration. With this visual feedback the player gets to see, how Gray views his surroundings. This is maybe one way to convey his biased outlook on reality.