Yeah, there are several things that could be called "trunk panel". But I'm guessing you are referring to the center area of the aluminum trim panel on the deck lid.
This is not as simple as question as it might seem. The default answer is "Argent Silver", but Mopar had more than one shade that they called "Argent". What you usually find as "Argent Silver" is close to Krylon Dull Aluminum, but that is way too bright for the 67 trunk panel and the centers of the taillights (which should be painted at the same time). Looking at the taillight centers should help, because this paint on the chrome doesn't fade nearly as bad as the paint on the aluminum. Of course, these pieces were painted separately at the factory so may not have been a perfect match even in 1967. Many taillight centers are dark — some look like they were charcoal or even black — but it is hard to say if they had been touched up over the decades. The 1967 new model brochures show at least two different colors on these panels, but the photos in these documents were shot months before any production cars came off the line — they were prototypes.
"Dark Argent" as seen on later rally wheel centers and other Mopar models is probably more like the correct color. I used a Metallic Charcoal touch-up paint from the auto store that seemed plausible. You don't want to lay it on too heavy or it will bury the very shallow texture that is etched into the aluminum. You want to just dust it on lightly.
Here is a photo of my unrestored car in 1985. See how the center panel is bright — the paint has almost entirely faded away. The taillight center is darker, kind of medium gray.
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This is a recent photo showing the color I painted it.
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Depending on the color of your car, you might want to go slightly darker or lighter. The main thing is, I don't think there is a definitive answer.