As you can see this opened another menu for you to utilize. For basic use click on the right hand colour ribbon on or about the colour you are looking for. Then click in the body of the square gradient to more fine tune the colour you like.
For more advanced users you can add the RGB, HSV or Hex colours if you know exactly which you would like to use.
Play with the lighting until you find a colour that you are comfortable with and then proceed to the next part.
Ok, ready? Now we will start casting shadows. By default in a new environment you will have the sun directly above you. This means it is not going to really cast much of a shadow. We will want to rotate the sun direction to either match the sun or moon or other light source in the skybox. To do this, use the sliders for the sun's vertical and horizontal axis.