For my Warlock concept art, I used "photo-bashing", where I take images and merge them into my art in order to create texture/patterns. The jacket, collar, and helmet are the spots I used it the most, but you could also tell by zooming in or looking closely. This is my first time trying this technique, I'll definitely be using it more down the line, but I should probably check out some tutorials because I'd like to improve!
As for the theme of the Warlock, I wanted to make a scavenger-like dogfighter pilot.