Set numeric values by dragging in Maya

Software:
Maya 2018

Hold Ctrl + MMB Drag the value field.

Simple Toon shader setup in Maya & Arnold

Software:
Maya 2018

Use a Surface Luminance node as a V coordinate input value for a Ramp (V Ramp) node containing the toon colors.

* Note that the Clamp node is important so results will be ass expected, it clamps the Surface Luminance values to 0.99 so the V coordinate will nor cycle back to the beginning of the Ramp.

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Related:
Toon Shading in V-Ray for Maya