Jim Benjaminson is an officer in the Plymouth Owners' Club and a noted Plymouth authority. One of the books he wrote is 'Plymouth 1946-1959' which is a fairly complete history of those years. It's available on Amazon.
It might be easier in the long run to bring your existing radio delete plate to an automotive paint supplier for them to match the color electronically. I've heard that some original paint ingredients aren't available anymore, so you'd end-up with a modern formula anyway.
It doesn't appear in my signature, but I also have a '51 Business Coupe that hasn't been registered in 50 years. It's pretty much complete (thanks to several other cars!) but has major floor and rocker rust. I shouldn't be too surprised because, according to the build card from Chrysler, it was originally delivered to a dealer in Detroit.
Keep us posted on your progress.
Regards,
Harold