Use quick and dirty clay models.
I use clay models when I am working because I have a lot of trouble remembering which direction my light source is coming from.
Post a copy of a color wheel on the wall.
Whenever a shadow is cast on, say, a red surface, the shadow isn't black. It is gray with a certain amount of the "complement" color (or "opposite" color.)
A shadow on a red surface has more green in it, because you are removing the red.
But I can't remember complements ... ever.
So ... I have a color wheel posted on the wall.
When I need to remember the complement of a color,
I just look up and see what color opposite is.
Works for me!
Have a great weekend! Carolyn