3D Posterization at Render Time
As an alternative to individually adjusting your various materials you can do a quick posterization adjusting the 3D Posterize slider found in the Render > Render Properties sub-palette. Increasing this slider’s value will increase the posterization effect.
The benefit of this solution is that ZBrush uses the surface curvature rather than color to establish the posterization. So when PolyPaint is applied the posterization will not be affected. This posterization is done on the top of the existing material, rather than altering the material itself.