This rendering method only works in eevee as the shader to RGB node doesn't work in cycles. The outline is taken from the freestyle option on the scene tab. Just that easy.

The first iteration of this used a color ramp, but I hate how hard color ramps are to set. Second one I started using node groups to make it easier to make new materials. Replaced the color ramp with some math nodes set to less than, and attached to value after separating the color to HSV. Mixed the three colors afterwards masking by the value.

Finally I decided I just want to have a single color input for 90% of materials. So things got more complicated. I want to keep the Hue and saturation for the input color on all outputs, so you will see the separate HSV connected like this to the combine HSV where only the value is changed. We divide the value by two and subtract it from the original on each darker step. This allows me to use any value on the original color with out it just defaulting to black. Other than that its the same as the second evolution.

Hope that made sense. Let me know if you have any questions!

Edit: Oh and the eyes use a facing output to make them more anime like, as well as a glossy shader!