How to paint within a boundary of given positions

I am making a coloring app where I am getting the face of the user with Dlib Face Detection plugin and trying to color on its different features like the eyes, lips etc.

Now the face detection plugin gives me a bunch of positions for the different features of the face. How do I create a boundary using those points so that my color does not go out when coloring ?

For example, on coloring the lips I want the coloring to stop if the users mouse pointer is out of the boundary points for the lips provided by the plugin.

Kindly help on this as the project is urgent. If you need any more details please let me know.