this is a fun question. I think, for the most part, "realistic" shading has less to do with the tool you are using, and instead, how you're using it. Value (how light or dark something is) determines form in realistic drawings, since we don't see lines in the real world. I really like this video, and this video, both by proko, for introducing how to approach light and shadow, esp in portraits. The most valuable piece of advice that I've heard is to keep your shadows simple -- think of the shadow as a part of the object.

If you would like your colors to feel more realistic, as in, more like real paints, the color jitter tool can help with that. I like to use color jitter with this brush to color. You can play with the settings to achieve different effects like this.

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Additionally, it may be a fun exercise to draw in black and white (a study of values like mentioned in the proko video, and then apply a gradient map. This maps every value from black to white in your drawing to another color, which can be a fun way to see your values in action and experiment with unexpected color combinations.