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[Patrick Gaumond]

Shadow colours are tricky to work out. What paul and elderscroller have said so far is good advice, but I'm going to expand it a bit.

As scroller already said, the shadow colour is going to be determined in great part by the ambient light source. If you understand how ambient light works, you can make better predictions from imagination of what the shadow colour will be. It's very, very hard to get it 100% accurate all the time, especially the further you stray from natural light sources like sunlight.

I made a little chart to show what I'm talking about. For instance, inside. If you could create a space that had 0 reflectivity, then there would be no shadow detail. This is what you see in outer space, since reflected light is very weak it cant bounce around from planet to planet to create fill lights and illuminate the shadow sides. I don't know of any surfaces that reflect no light. You need some reflection to see the object, as light needs to enter your eye for you to see it. Even matte surfaces which are defined as surfaces that do not reflect light strongly still reflect some of it.

Adding in a second light source reduces the darkness of the first shadow, and softens both shadows. As you add more and more light sources, shadows become less and less pronounced. This is what "ambient light" refers to. The ambient light from the sky is essentially like having infinitely many tiny light sources, but they only affect shadows because the light from the sun or the primary light source is infinitely stronger than any ambient source could ever be.

It's a bit crude by I hope it gets the idea across



Take those ideas outside and you get something a bit like this. If your object is close to a surface thats reflecting light (ie the ground in this case) it will most likely take on some of that colour somewhere in the shadow, as well as any ambient light from elsewhere





Hope that helps!