Light Sources

In the second chapter of his book "Color and Light" James Gurney talks about sources of light, what's generally easiest, and how to approach each situation.

Direct Sunlight:

The sun is the greatest source of light, then there is blue sky and reflected light. The sky and reflected light should always be softer than the direct sunlight.

Overcast Light:

Overcast is generally easiest - the colors are straightforward, and there are no drastic shadows. 

Window Light:

This is popular because it simplifies the light and is constant. 

Candle & Firelight:

Yellow-orange in color, this is a weak light that drops off rapidly. 

Gurney mentions a few others as well, but these are the ones I liked the most.