>>7593205
https://x.com/danhollick/status/1583080119068807168
If you want a technical breakdown of what the modes actually do computationally, here. But tl;dr the multiply mode just allows you to shade things without having to think too much, useful for cel-shading flats. Because they're a separate layer you can go back and edit it as much as you want without ruining what's underneath as well. As for what colors to use, depends what color shadows you want to end up with.
Here's a video that explains shadow colors and how to choose them:
https://www.youtube.com/watch?v=gwLQ0cDb4cE