As for my  nissan laurel  1993 vintage the engine compartment is the same as the body colour , but without the pearl top coatings, just a sort of white in my case. You may find that the metallic and pearly finish coats are omitted  as there is no real advantage having it there.

according to the diecast version, yes the engine bay is body color

A lot of modern cars do paint the engine compartment and underside the same as body color. I think this has to do with being unibody construction. Of course depending on the region the car may have an underbody protective coating applied which may be grey or black.