how are old paint colors figured out ?

Well have posted before about trollin' for a '64 dart gt convert, and getting it to be like my old one was which means repainting it. Mine was this ivory sort of color (w/ black trim), which appears to have been a stock Dodge color for that year. I actually found an original Mopar paint chip chart, and the color is on there, but sure could have faded in nearly 50 years.

I guess the question is how do paint shops figure out how to match a color that hasn't been seen in years?