Okay, with the helpful comments of a couple YouTube viewers, I figured out how to do it. I’m posting here in case anyone else has the same question. WAY easier than I expected, once I knew what settings I was looking for a tutorial on.
I’m not sure how to do this in Cycles. I’m sticking with Blender Render engine till I get the hang of things because the settings are easier. But if you understand all the complicated Cycles nodes stuff, this should be easy to adapt.
Under the Materials tab for your object, select the following settings:
Base Color: Base color of object (in this case, green)
Ramp: Check! <— THIS IS THE IMPORTANT PART!
Color 1 (position 0): Shadow color (blue) at 50%
Position of Color 1: 0.33 (one third of the way up the ramp)
Color 2 (position 1): Base color of object (green) at 50%
Position of Color 2: 1.00 (top of the ramp)
Input: Result <— ALSO VERY IMPORTANT!
Color: Color of your main light source (in this case, yellow)
(Intensity and hardness will of course change with the material of your object; numbers for skin will be lower, metal will be higher, but 0.50/25 is a good starting point.)
(Another possibly easier option is to completely turn Specular off [Intensity = 0.00] so that your object’s tone is altered by the color and brightness of your light source, but there are no defined highlights. Then you can go in and paint the highlights according to your own style. Upside is less settings to worry about and more control over the color when painting the highlight in; downside could be that you still need to understand how to paint materials [metal vs. skin vs. fabric, etc] to reproduce the correct specularity by hand. I didn’t do this in the example below, but by the end I wished I had. I wasn’t happy with the highlight on the sphere.)
Translucency (under ‘Shading’): 0.50 <— THIRD SUPER IMPORTANT THING
Here is the final render with edges and a yellow main light, a blue fill light, and a red rim light. (If you aren’t sure what these terms mean, look up tutorials on “3 point lighting system.”)
And here is the final painting:
Never say it can’t be done.