Color and Light - Ch 2

There are three main sources of light on a clear sunny day: The sun, the blue sky, and reflected light from other objects.

Shadows become bluer when it is sunnier and grayer when it is less sunny 

 In this picture the shadows are shown to be really blue and purple to contrast the bright yellow sunlight coming down 

 In this one its an overcast day so the shadows and tones are more grey and muted. 

Overcast lighting best captures colors because it doesn't have high contrast that comes from shadows or direct light. Its used a lot in photography. 

 Overcast days also work well with portraits because it doesn't cause a lot of contrast in the skin color. 

Light will reflect what it is bounced off of, so light coming in through a window may have a blueish tint from the sky or a greenish tint if its bouncing off of grass. 

In this picture the light from the ground is being reflected onto the ceiling, showing a warmer color, where as the light from the sky is being reflected below, showing a cooler color on the ground.

Moonlight will make things look blue or grey

You can use cool and warm colors to contrast against one another to create more visual interest and contrast.
This seems to be done especially in movies.