The folds of clothes are drawn by contrast between light and shade. The clothes you draw will have a hue, and then the concave parts of the folds will have shadows-painted in a darker color than the clothes, and the prominent parts will have highlights. The shape of the fold should be as curved as possible, just be casual and natural.

1. If you can't adjust the color well, you can use the "Deepen and Lighten" tool in the tool.

2. You can search online for hand-drawn pictures with clothes wrinkles, imitate the shape of wrinkles, or draw the color of the original picture on you with a straw.

3. Or search for some sketches of figures in art, or hand-drawn illustrations, which reflect the folds of clothes.

There are three relationships when painting: light, gray and dark, which can be expressed through appropriate transition. I wish you success ~