Here's a tutorial on how to create a simple toon material, plus advice on other more advanced options available in Blender and 3dsmax.

Reference

Software Agnostic Material

The basic ingredients are as follows:

• Shading: Instead of a smoothly shaded diffuse surface that goes smoothly from lit to unlit, you want the colors to pop rapidly between levels of dark and light.
• Outline: A black outline around the edges of objects as well as the interior edges of surfaces
  
• Color: Whatever you choose

• Many hand inked pieces of art vary the ink width based on factors such as illumination. So the areas of the model that are in shadow get a thicker outline, and the areas in light get a thinner outline. Doing this with your Toon Shader will make it more realistic.
  
• You can also add some texture indications, like for example the stylized dirt on the spacesuits in the reference above.
  

Note: This example requires the EEVEE renderer.

Here's the basic shader for Blender...

First notice this doesn't contain a Principled BSDF, instead we're hooking the color directly into the Surface of the Material Output. Then we use a Diffuse BSDF node to capture the lighting in the scene, hook that into a Shader to RGB node to convert the shaded values into rgb values, then use that inside of a Color Ramp to control the colors.

In the Color Ramp, instead of a single color, choose whichever colors you want painted on the interior of your surface. Then set the value to "Constant" so you don't get a smooth gradient, instead you snap between the colors you've chosen. Move the tags in the gradient to get the values placed on the surface any way you'd like.

For the outline, there's a number of different method.

Method 1:

This method is the one shown in the shader above. Use a Layer Weight node that will capture the edges of your surface (sometimes referred to as a Facing Ratio), set Blend to 0.8, hook this into another Color Ramp, and adjust the color ramp's gradient to create a sharp black outline at the edge. Then take the edge nodes and the painted surface nodes and mix them together with a Mix node, using the outline as a mask to reveal the color Black that's in the Mix node. Hook the mix into your Material Output.

This technique only works if your objects are rounded, like a sphere, cylinder or an organic creature with lots of small faces.

Here's the resulting render.

Here's the blend file, Blender 5.0: toon_blender_01.zip

Method 2:

An alternative way to do your outline is to go to Add > Grease Pencil > Scene Line Art. This adds a scene object that adds an outline to everything in the scene. Select the LineArt object it places in your scene and go to the modifier panel, the two parameters you'll adjust the most are the Line Radius, and to capture more edges in your outline, increase the Crease Threshold value to a higher number.

This technique will only look correct when you look through the camera, not an orthographic view. And it works much better for hard surface objects than method 1, things such as cubes, boxy robots, and vehicles.

Here's the resulting render.

And here's the same technique used on a far more complex model:

You can also adjust the line thickness based on illumination by applying a Weight Proximity and Thickness modifier to your Line Art object. Adjusting the Lowest and Highest values in the Weight Proximity modifier to adjust the maximum and minimum width of the line. Thanks to julperado for this trick!

Here's the resulting render.

Here's the blend file, Blender 5.0: toon_blender_03.zip

And finally method 3 is for those of you who are really serious and want a professional plugin. Checkout Psoft Pencil+ here: https://www.psoft.co.jp/en/product/pencil/blender/ It's expensive, but it gives you a crazy level of control over your toon shader, and has been used on lots of major anime in Japan. Thanks to Thorn for the suggestion.

3dsmax (scanline) Example

To create a simple toon shader in 3dsmax, we'll use the Falloff Map. A falloff map is basically a way to vary a color on a surface based on a series of functions. One such function is called Perpendicular / Parallel, which blends between two colors based on the surface normal direction, so edges of an object can be a different color then the front. Another function is Shadow / Light that lets you change color between the dark and lit part of your material. Using these maps, and layering them together using mix, you can make a color layer that's based on Shadow / Light, a highlight layer based on the same thing, and an inked edge based on Perpendicular / Parallel.

The key is to adjust the output graph on your Falloff, making the transition between colors very sharp, to get that hard edge transition between bands of color.

A similar output trick is used to get a sharp edge, and a sharp highlight.

Also note the material is 100% self illuminated, so that you get the pure color with no diffuse shading.

For complete details on how the material works, please check the file.

Here's the max file, 3dsmax 2022: toon_3dsmax_scanline.zip