Maybe if you use a darker oil pastel to create the shadow in the knee join to block it in more. I would focus on creating more 'shadow' blocks because most of ur dark colours are in the line work, additionally, I don't know abt the reference photo but typically there are more folds of skin around the end of the butt cheek area, so it's less of a defined line and more a fold. It looks rlly good btw
