This is a problem I’ve encountered quite a bit. Basically, the model is in shadow, when there is nothing in the scene which indicates that it should be so.
- Restart Zbrush.
- Load a model
- Do a quick test render (Shift + R)
Keep in mind that the light and the render settings are exactly the same here.
Workaround: Save the model as a tool, restart Zbrush, set the size of the document (you may want to change the size if you for instance want to render out a turntable video), then load the tool. This should work.