Hi John,
I ran into a similar issue trying to match the original medium blue interior in my dad's 77 GMC. It wasn't listed in the SEM color chart (or any others).
So I sent an email to the SEM tech department asking if they could color match a sample. They said they could so I mailed them a piece I cut from a spare door panel from a parts truck with the same color interior. A short while later they emailed back a color formula. I had a can of SEM ColorCoat mixed at the local paint supplier using that formula and it was an exact match.
If you don't have any old junk/spare parts to use for a sample, take a look under the seats, at the backs of the door panels, and such. There might be some extra material you could cut from a hidden area. Just make sure to get a sample from an area that isn't likely to be faded from sun exposure. On the door panel, I cut the sample from an area that was protected behind a trim piece.
