On the F-86f-30 hardwing Sabre, there is a spanwize centre piece to the upper wing surface that is not the same “colour” than the rest of the natural metal.
I can’t quite tell from the pix, is that section light grey painted or a different tone of bare metal?
The center wing structure of the Sabres was not likely paint, but a coating that made it look darker than surrounding panels. I usually paint my models with Alclad “dark aluminum” but add a bit more black to it to darken it more. This is a Hasegawa 1/72 F-86D Sabre Dog with Alclad finish.
I remember reading recently that this portion of the wing skin was made from a different metal. But I do not recall which one off the top of my head, nor where I read that tidbit of information… Sorry I can’t help more right now.