I am using zbrush 4 and assigned a subtool with various polygroups just one polygroup whilst in the show topology mode which shows the polygroup colour and the lines of the mesh. When I clicked out of that mode the model now is completely black in silhouette in the viewport and you can’t see the mesh or material. I remember this happening to me a while back but have no idea how to reset it so that I can just view it with its normal red clay material does anyone know how to undo this?
letter C 
First, make sure that it isn’t just your color picker set on black. If it is set to white then do the following.
Turn on Mrgb button. Select Flat Color material. Go to Color menu and click FillObject. This will reset your material. You can now pick any material you want.
Setting flat colour and fillobject sorted it out - no idea how I got it like that in the first place many thanks