Brainstorming, I generally do this process before begin to work in the actual piece

Here I was testing my polygonal brushes for the "Lowpoly" effect that I wanted for the image

First block of values using a mix of the polygonal brushes and a soft brush

This is the coloring stage, I just blocked some initial colors to have the general mood.

Added some textures to make it more realistic. The overlay mode is fantastic to create more saturated parts to enhance the contrast between the colors.

According to the colors I painted the persons "personalities" fighting for the control of the mind.

Final illustration with some color correction and post production effects to improve the look of the illustration.