Paper or Plastic?

In this composition I attempted to capture the different contrast and texture of three different paper bags. One is crumpled up, one bag that is darker, and then one that is a lighter shade. In this piece I used charcoal. I absolutely love using charcoal and pastels for pieces like this. I feel I did an alright job at capturing the difference in the hues and shades of these bags. However, I'd love to perfect this technique. If any of my fellow artist could comment with any pointers or any constructive criticism would help tremendously.