FWIW, there are several photos of the NP 90000 series refrigerator cars in FOFC Vol. 8. I see a distinct tonal difference between the "kick plate" and the door hardware, which was black. I'd say Bill got his models right, and the kick plates were freight car red.
I don't have any color photos of these cars, at least in the era I'm interested in, pre- 1950. I'd welcome any definitive proof as to the color of the kick plate, when it is painted a contrasting color from the yellow side.