how are old paint colors figured out ?

The paint code should be on your fender tag. If you have no fender tag, check some references that list the available colors that year. That info has appeared in FABO threads a lot over the years. Do a search on the site for fender tag decoding, paint, etc. Also, there is a website that collects a lot of Chrysler historical data; I think it's name is Hamtramck Historical something-or-other.

P.S. Check here:

http://www.hamtramck-historical.com/

http://www.hamtramck-historical.com/library.shtml