Perhaps the theme of harmony or imbalance. If everything is green, is it in a state of perfect harmony? Or is there too much of one thing (the color green) which represents an imbalance? The story could be delving into how a single - colored world affects the characters' perception of beauty, normalcy, and their own place within this unique environment. It could also touch on the idea of monotony versus variety, as a world full of just one color might seem monotonous at first, but there could be hidden depths and nuances within that sameness.
One possible theme could be nature. Since green is often associated with the natural world, the story might be exploring the relationship between humans and nature. Another theme could be envy, as mentioned before, with green symbolizing this emotion.