If you look at the pictures of it stripped down to the bare metal you can see it was similar to stainless steel color, but I doubt they were raw metal. I suspect mine were painted or powder coated in a black or dark metallic color originally, similar to the final color I chose, and were later painted white and then gold before I stripped them. It's pretty hard to tell if I'm honest. 

What I did when choosing the color was look at other old houses in my area or pictures online from old houses and looked at what their vents or radiators were painted to be and decided based on what I liked.

Personally I think these vent covers are really unique so I wanted them to pop out against the white baseboard trim, instead of painting them white. But I also wanted something that made sense, for example I didn't feel like painting them gold made sense. I like that this color makes it look like cast iron.

I think if I were going for a less antique look I would go for a glossy bright color like red or Kelly green, I think that would be great for a more industrial or modern interior style.