Good sketching it’s not that easy
If you’re sketching to prepare a street production, you’re free to use every kind of trick. Few black & white lines can be enough: the rest of the idea is drawn in your mind ready to be used when needed. But if you’re sketching as a form of training or fixing your style, the paper becomes your street and the game is much more difficult. How many years will your sketch survive before you will judge it outdated?