What are the any 2 reasons why a shadow is light?

Your question is poorly worded. A shadow is not a thing to have some color, it is merely a pattern that you see, formed by light striking a surface but blocked in part. So if another light shines behind the block, the shadow looks lighter than you expect. Snow is illuminated by light from the sky, so it looks sky blue instead of dark white. There are plenty of places where you can see colored shadows caused by colored lights. Like this:
www.google.com/search?tbm=isch&q=shadow+on+snow
www.google.com/search?tbm=isch&q=colored+shadow