Cadillac Mike
Recommend you get on the Society's website (nycshs.org). Color info is in two locations: In a top, pull-down tab under "About NYC" there is an article on NYC colors, with qualifications but also including the paint formulas used by Lowes, etc. I am aware that at least one O- gauge/scale importer used this color information for a postwar 20th Century Limited (with no complaints of which I am aware re the color).
Under another pull-down tab "Modeling Technical Resources", go to "Modeling Technical Data" for additional and more contemporary paint information.
As info, the best color image of P&LE green, and the image used by MTH when they issued the P&LE A-2A Berkshire, was a color image of #9401 taken by the company photographer of the clean engine when it pulled a Directors' special when the engine was new. NOTE: This MAY or MAY NOT be the color used by P&LE for painting cabooses.
Finally, from my personal experience with a locomotive builder, I do know that exact color was not closely controlled by the railroad industry, at least until the late diesel age with the advent of sophisticated color check equipment, and color varied from shop to shop on many or most railroads. Perhaps the best example of this is to view a color image of a freight train and the wide color disparity of "box car reds" in any freight, even allowing for weather related degradation of individual cars.