Here’s an illustration of the point, “On a sunny day, vertical surfaces in shadow usually receive two strong sources of illumination: warm ground light and blue sky light.”
The shadow side of this pile of snow is distinctly blue on the top surface. The horizontal plane of the of cast shadow at right is also very blue from the sky above. But the vertical surface in shadow is receiving a combination of that blue skylight and a warmer light reflected from the illuminated surfaces of snow on the ground.
Reflected light influences not just shadowed surfaces, but also illuminated surfaces. Just to the left of the base of the pile, the snow on the ground becomes very light where the reflected light is added to the direct illumination from the sun.
Color and Light, page 67.