One advice I would offer to people learning to draw would be to experiment with the materials. Drawing is a separate skill from mixing colors or from knowing how to use a particular artist’s material — colored pencils in the example above.
You can separate out the tasks, like color mixing, and gain mastery of the parts in ways that prove helpful later when tackling a whole drawing. For color I think you should mix everything with everything. Ignore the advice that says “only do this” or “only do that” and instead just go crazy. Especially ignore the “never” advice.
Are you never supposed to mix more than two colors together? Mix five together — or ten — more more. (Try not to wear a hole through the paper with all that rubbing.)
The main thing about color and color mixing is simply to notice what happens. Be very alert to the visual properties of whatever mixing you do. How does mixture or juxtaposition change the character of the colors? Particularly important is how does the melange of colors make you feel? Color can by-pass reason and zip right straight into your head, affecting your mood. Learn not only what combinations create this or that color, but what combinations create this or that emotion.
Someday, then, the artistic problem becomes more interesting: how to mix the feelings ….