Render a selected part of the terrain with different shading

I'm trying to understand how you can change the shading of the terrain section based on a user selection. When a user selects a city or entity in a game like Civ5, that area is more illuminated than the other parts of the terrain. I try to implement this effect.

In the Civ 5 example, the selected tile has a circle and the selected city has a transparent overlay that closely follows the elevation of the terrain. Civ5 screenshot

Square shading of flower pictures – MathOverflow

I wondered if anyone could help shade these petals. It's half finished, but I just want to shade the inside of the petals.

This is the picture:

Enter the image description here

Here are my previous equations: Enter image description here

I wondered if anyone knew which domain and area restrictions I could add to just shadow the inside of the petal.

Many Thanks