The skin shadows look rather dull, making your people look plastic rather than flesh. Instead of using a simply darker colour of your skin tone, make the shadow a little more red or blue ("hue-shift" it). That'll add a bit more life into the colours. Red is generally a safer option because no matter what the lighting, skin always reflects some red. Blue can look more interesting, but looks out of place in some situations.
I've been mixing blue with a darker version of the skin color for shadows for my illustrations. Maybe I will try mixing red next time.
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