I understand the tone of the world you’re going for. You might want to pump up the gray for the dark world so that when you have green for the plant, it stands out more. My suggestion would be to dial back the saturation of the colors and pick colors that are more on the opposite side of the color wheel from green. Keep those colors feeling unnatural. When people take their helmets off, make their skin feel, grayish and not red. Blood is what carries oxygen, and you should make people feel like they’re bloodless, or lacking in blood. Like sad vampires.