It blends colors. A little tricky at the start, but does wonders when you master it - and I still haven't, I guess.
Just paint with a brush or pen where you want to be dark in your pic with a contrastant color and blend it with the rest of the colors, starting from the limits. If this sounds confusing or you don't quite manage to get it right, I can make you a very basic tutorial. I find much easier to explain with drawings. :b