The most common mistake of beginner sketchers is painting shadows a bit too light. To get better, practice by painting small thumbnails to achieve the right contrast using only tonal values. Once you're comfortable with achieving the appropriate contrast with just black and white, you can move on to colours.
Reference photo:
https://c1.staticflickr.com/5/4488/26329445669_db7eb4e0ac_k.jpg
ArtGraf review:
https://youtu.be/kNf9Q0cgmsE
Graphitone review:
https://youtu.be/YFdjYOOUPPI
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