The colors look flat because you can't decide where the light comes from. Try to practice shading basic forms like spheres and boxes with 1 light source - it will help you immensely with shading. Try using hard round brush for painting shadows of the objects that are close to each other (e.g. elbow area). You also use way too light and saturated colors for this sort of lighting. Always use white and close to white colors sparingly. I would also highly suggest observe real life photos. Find any in the same light setting as your drawing and color pick skin tones, shadows etc. You will notice that skin often is not just red or shadows darker color than mid tones. For visual interest: I feel like the clouds are too low and fight for attention with the main subject. I also added some sort of buildings in the back so it is not that empty. | Artwod Feedback