mantelpiece

Hi everyone. We had our fireplace opened up recently and a second hand marble mantelpiece fitted. The mantle shelf has been fitted slightly higher than the surround, which has left approximately a 10mm gap. Builder tried to fix it with some shims and caulking on the sides, but it's all visible...

I attach an image of the part of the mantelpiece that needs fixed with a lick of paint, I'm not sure of the material but going by all the types available I think it is metal but thought to include an image to see what you think as to which particular paint to use. I don't want to have to paint...