Symptom: In the rendered image, some of the elements display as gray. They do not display render appearances as expected.
Issue: This problem occurs when you upgrade a Revit 2008 (or an earlier release) project that uses custom materials. During the upgrade process, Revit upgrades all standard materials to use the new render appearances. However, it cannot upgrade any custom materials.
Solution: Assign render appearances to the custom materials. See Changing the Appearance Properties of a Material.