How would you do shading using acrylic paint?

I mean like if you want to paint something light green at one end, dark green at the other, what technique would you use? Or for green shading into blue, or whatever.... I have my own ways, but I'm interested to know how other people work.

The look that particularly interests me is when you see pictures of people and their skin is lighter around the edges, giving a "luminescent" look I see this look a lot. I find myself shading things simply by applying the paint lightly where I want it light and more heavily for the darker look, same paint, no "mixing" involved, although I suppose the white of the gesso or paper underneath is kind of doing the "mixing."

Anyway, how do you shade things? BTW I know oils are meant to be better for shading, but I'm still not 100% sure why...