Thanks @dale, hadn’t noticed the introduction of RenderMaterial. I’ll rework to accommodate the update.
I still have a question though. How come Materials.Find(“Orange,” True) still returns -1 despite having been added to the document?
E.g. in a vanilla Rhino6 doc., Rhino.RhinoDoc.ActiveDoc.Materials[0].Name returns Plaster, but Rhino.RhinoDoc.ActiveDoc.Materials.Find("Plaster", True) returns -1