Ditto on the question as well. Some of the nicest cars I’ve seen restored were engine color. I don’t know if it’s correct but, need to paint one for mine as well.
I looked around a bit and it seemed like engine color was the most common. curios as well. Thanks for posting the question.
This article I just found says the cover is natural, but painted with the bell housing and engine. This means it would have had overspray, not full paint coverage.
Of course it's up to us whether we go with the factory method, or make it look nice with a full coverage paint job.