Great questions! To really understand what changes need to be made, the first step is to find out exactly what the differences are between those materials. For this I don’t think we’ll need anything other than the principled shader and some textures. Take a look at some references online of painted metal, plastic, and rubber.
You’ll notice that besides the patterns on the surface, there really are very few factors that are different. In this case, roughness, SSS, and perhaps a sheen or clearcoat on top. Light either gets absorbed, refracted, or reflected by a material, and these settings tell Blender how much of each should be happening. For a deep dive into how this all works, check out these lessons on PBR shading: https://cgcookie.com/lesson/pbr-introduction
Since plastic is a well known substance and our setup is based on its physical properties, there are really not that many ways to set it up correctly. Instead, try adding a grunge texture to the roughness, scratches to the normal, and an indoor HDR texture for the environment. Also be sure that you’re using Filmic color management, so that the bright areas don’t become oversaturated.
Give that a shot and let me know how it goes!
