To produce stylized renders, ZBrush provides flat type shadows as an alternative to the classic gradient shadows. These flat shadows are monochromatic, providing a cell shading style for your models.
Flat shadows can be used both in the real-time renderer while sculpting and at time of final render.
To enable flat shadows during a BPR render, turn on the Render >> Render properties >> Flat Shadow mode. Shadows must of course be enabled to have access to this setting.
For real-time flat shadows, enable the Render >> Preview Shadows >> Flat Shadow mode. By default, the Vibrant Shadow mode located in the same sub-palette will also be enabled. For a better NPR look you will most likely wish to disable this option.