When rendering an illustration you need to think where is your light source and what qualities it has. In that particular example I would assume it's moonlight. Then the light should come from above, be dim and cold in color. Local color of everything would not be as clear. The green skin color would be even more grey in shadows, in fact every color would not look like you think it should. you can notice it yourself by trying to identify colors at night in your room. Night scenes are pretty complex and I would recommend you to repaint this image but in daylight. When doing so break down every element in your image into simple geometrical shapes and try to imagine what surfaces would be hit by the light and what would turn away from the direct light. I would also recommend you to practice shading in black and white for a while before doing shading with color. And check software called Colour Constructor 2 to see how much color can change based on how well lit it is and the color of the light Good luck!