UPDATE:
While awaiting MTH to supply me with the correct paint chip samples for the Showroom cars, I dug out the paint chip chart that was supplied with the TCA "Standard of the World" book (second edition). Although the modern MTH tinplate colors are NOT an exact match for the original Lionel colors, even when those finishes were new, they are fairly close.
I might just take these chips (light and dark state green) to my local paint shop and have them custom-blend a glossy enamel for each color.
The light state green is definitely easier to match with commercially available paint chips, and I found one that is very close.
MTH CAN supply the two-part (hardener + enamel) for these colors, but these have to be mixed in a certain ratio, and then quickly applied. Also, the shelf life of both hardener and paint is very limited.
I have declined that "solution" in favor of having my own colors mixed. With an opti-visor and a fine camel's hair brush, I should be able to paint over the primer carefully enough to have the touch-up look fairly decent! 
