What is the correct color for 67-68 lower door vents

The vents are supposed to be color keyed (same color as surrounding door paint) but... there were several different shades, hues, and textures of reds in these cars. The door metal will be a medium gloss paint while the original vents were molded with red ABS which cures flat.
Its not easy to change black to red with any type of paint. When dying vinyl or plastic, for best results I've found to first go to a neutral color like sand or camel beige. Then cover that with the red. Back when I worked as used car PDI at a Ford dealer, nearly very plastic replacement part arrived in their camel beige. If I got lucky I ordered for a big ugly green Thunderchicken with the camel beige interior. No dye req'd , pop it in and go.
Back to your vents... Factory could have gotten a more gloss finish from the molded ABS with a oil cooled mold but slower cooling slows production. Black ABS for plumbing was oil cooled, not for the shinny finish but for the rigidity requirements.
Yeah I know too much about some chit due to my job hoping over the years. Plus my owning 2 of 67 mettalic red gut 67 models.
If you want a concours correct look to aftermarket black plastic parts like your vents., take the little column door from beneath the steering column to a SEM dealer near you and have it matched in a RTS. Don't forget to pick up any neutral base/primer shade in a spray bomb off the shelf.