plastic paint???

Tops of the rear panels should have the same finish as the tops of the doors. Its a medium gloss. To get that from SEMs you would need to clear coat it.
I choose to use 2 different paints on these parts. SEMs on the textured lower section, same enamel on the top that is used on the door steel.
If you remove the chrome trim from those rear panels you would find the factories paint line behind it.