Rendering of a selected part of the field with different shadows

I'm trying to understand how to change the shading of the terrain section according to the selection of the user. In a game like Civ5, when a user selects a city or unit, this area is brighter than other parts of the field. I'm trying to implement this effect.

In civ instance 5, the selected tile has a circular outline and the selected city has a transparent layer that follows the height of the terrain. Civ5 screen capture