Here are the 5 important things about painting shadows.
There are two types of shadows. Cast Shadows and Form Shadows. Cast shadows are the ones which are cast or fall on some object other than the object that is casting it and Form shadows are the shadows that are caused by the object's form turning. These shadows are on the object itself.
Bright Light causes Dark Shadow and vice versa. On sunny days the tree shadows look dark and on cloudy days there are light in value.
Position of light Source defines the length of the shadow. In the morning and evening time, the shadows are much longer than during other times of the day.
Lit areas of any object are lit by sun and shadow areas of these objects are lit by sky and hence the shadows appear bluish in color.
Color of shadow is a darker version local color and complimentary color of light source. Never use black in the shadows!