They teach you to start with a black (dark) room and add your light - one at a time. It wasn't until recently that I learned why I like oil paints, but hate watercolors. It's because in watercolors you start with a white paper. In oil painting, the first thing I do is paint the background (usually dark). This holds true in photography or rendering. Start with everything black, and then add your main light and get the effect you want. Then add a fill light. If you need to light the background, then add it. Learn how to paint with light.