Coloring game for mandala

I want to create a game of coloring book.
So far I have read and researched that I have to create the asset with different parts separated in white and then reconstruct them in unity. this is good for some basic drawing with large areas.
my question is is there a way where I will just have to create a png of the drawing with black outlines. User will tap on the empty area and that will be filled with color.
please help me out here.

This is done with floodfill. Google for more details about what is involved.

You can download a basic example of floodfill from my proximity buttons project.

The floodfill routine is in the ImageUtils class and the TestFloodFill scene shows how it all works.

proximity_buttons is presently hosted at these locations:

https://bitbucket.org/kurtdekker/proximity_buttons