Is the bumper plastic colored to match the paint?

I managed to scrape my rear bumper a bit and after using some very fine sandpaper to remove the scuff marks I still see a white base. I'm just wondering if this is the actual bumper plastic (colored white) or if I'm still in the white paint layer.

It makes a difference as to whether I apply a primer coast first or can just apply the color coat and clear coat. My understanding is that you need to "primer" the bumper material to get the color coat to really stick.

Thanks.