Update: Texture progress, handpainted in substance painter using hard/soft round and single texture channel. I'll be iterating on the metal and tattoos. Feedback appreciated.
HSV! EDIT: Just another thought. This would be perfect for people handpainting a game where you want to keep all the colours consistent for the same lighting!!!!
In the end, I just opted to using photobased damage like Racer shows in his excellent tutorial. Handpainting was arduous and didn't yield satisfactory results.