His brain is a big ball of electronics. To simulate that I used gesso, which is a really thick acrylic, to make dots or bumps all over his head. I made an extra large bump for his nose. After that dried I painted it black.
After the black undercoat dried, I put random dots of random colors of paint all over his head. His nose was the innards of an earphone speaker. That, the eyes and mouth were the only things I actually had to do a good job on. The rest was just sloppy dots.