How did they do these shaders/lighting? I'm a beginner. I've been using Blender for about a month. I've done a couple of tutorials with C4D and Blender.
I'm really interested in using 3D to assist my 2D illustration. I'm having a hard time because I don't see a lot of tutorials covering materials, shaders, and lighting to produce stylized images. It's usually about modeling or making something look ultra-realistic.
What kind of setup did they use to achieve these effects? I'm thinking of things like materials, shaders, lighting, etc. Where would I even start?
The image shown uses C4D:

Art by Andrew Footit
Yo guys. No more upvotes here, please. @ChristopherBennet and @RobinBetts both posted great answers below. Thankyouverymuch.

It looks like the color is based on viewing angle.

Add the Layer Weight node
Use the Facing output to drive a MixRGB node
Adjust the Blend value to control the gradient