I recorded a short video on how to find what you can change in Rhino materials. In short: save out the material to file, open it to inspect. Take note of the parameters you are interested in and then use https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Render_RenderContent_SetParameter.htm on your material instance, properly bracketed by https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Render_RenderContent_BeginChange.htm and https://developer.rhino3d.com/api/RhinoCommon/html/M_Rhino_Render_RenderContent_EndChange.htm
I hope this helps.