I found that using the burn tool (between the pen and blur tool) is an easy way to darken areas of your picture. When selecting burn, you can change to area of the burn to fit your application. It darkens the color that is being selected while keeping the shade appropriate. It seems to be much easier than selecting an area to change the color levels or selecting the right color to paint over. Using the blur tool after seems to make the end product look very nice, and realistic.