FWIW, mine are straight black...not metallic. If doing both, I'd suggest...and I don't have the code but it would be easy to obtain...using Ford's exterior straight black paint code.
It's not like they need to be blended, or match anything else....so 'd think any black would work and never be noticed as wrong etc.
That being said, Ford gives the secondary color codes for repair, but I don;t know if that includes mirrors. Perhaps a body shop can help. On my Excursion, I was able to get the code for the dark metallic grey cladding..which was available from Ford as touch-up paint.
Did the previous owner simply add the chrome caps, or swap the mirrors? Ford offers a chrome mirror cap accessory. That may be simply taped or clipped on. While I don;t know all your car's details..there's no shortage of SHO mirrors and caps on eBay.