I think you would need to do 2 storefronts/curtainwalls, one with one type of mullion (highlighted in yellow) and another storefront with a different type of mullion that represents the cover (highlighted in pink). Then you can specify different material/finish to each type of mullion.

If you don't want to spend time modeling this, you can indicate the color of the cover in notes.
As per Kawneer's documentation, covers can have different colors and shapes:

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