The single most important thing I learned in my color theory class in art school.
This changed everything for me in terms of understanding and utilizing color, light, and shadow in my art, so I hope it will be as helpful to you!
Basically the idea is that to believably convey light and shadow, you have to consider what color your light source is and shift the hue of the surface it’s shining on a bit in that direction. Conversely you need to shift the hue in the shadow region away from that direction. It’s much easier to demonstrate than explain, so just check out the video above!