Hey guys I hope you can help me out here.

I modelled a toilet, then applied the blinn material to the base and also gave that cap a material (also blinn) with another color.

Now I want to render the image, select mental ray as renderer and the problem becomes visible. The Toilet_base part has not the dark tone I set up in the hypershade. The toilet_cap renders just fine.

My questions:

What are the possible reasons?
Is it a geometry related issue?
Is mental ray fine with smoothed models?

and...

What can I do generally to avoid that?

I recently tried to render other objects too with mental ray and often I have noticed that the materials are not showing up in the render image, but in the viewport. (Sometimes with flickering or green faces)

Sigh... thanks in advance.