Note > The gradient must start with black and end with white, otherwise your image will not use the full dynamic range possible.

12.  The Gradient Editor dialog allows you to assign colors to shadows, midtones and highlights.  Click  underneath  the  gradient  to  create  a  new  color  stop.  Slide  it  to  a  location  that  reads  approximately 25% at the bottom of the dialog. Click on the color swatch to open the Select Stop  Color (Color Picker) dialog. 

13.  Cool colors such as blues or cyans are often chosen to give character to shadow tones. In  the project image the HSB values (hue, saturation and brightness) are adjusted to 208°, 19% and  35% (this brightness value is lighter than the 25% position of the color stop and opens up the  dark shadow detail in this image). Select OK in the Select Stop Color dialog to assign this color.