DuPont 503 was EMD’s standard car body interior color. I’m not aware of it being used on the exterior but anything is possible.

I wouldn’t be surprised that the gray was picked from a paint company standard chip color book.

The red was originally DuPont 1863R. From paintref.com, the PPG equivalent was 70073 or Martin Senor 109. The modern equivalents may be a bit brighter red i.e. PPG 907149 or 920321. PPG can probably still use the 70073 code and come up with the color. Swift Red was a very common fleet color. As long as you’re close, it’ll look OK.

The gray will be the hard part to get close simply due to the overall area - too dark and the red won’t pop, too light and it’ll look dingy.