From the Carroll Schmitt collection

I think everyone is missing my point. The B&W photos of the State of Maine cars establish the problem facing the photographer; Without a filter, both the red and blue stripe will print as almost the same shade of gray. To capture the colorful nature of the car, he must choose a filter. If we can determine which filter he chose, we can use that to decipher the placement of the colors.

This web site has a very good visual aid of the effect of different filters on ALL the colors in the composition:
https://www.photographymad.com/pages/view/using-coloured-filters-in-black-and-white-photography
Page down a bit to the chart.

We can see that the filter lightens like colors, but also affects the other colors in a predictable way.

If the top stripe was RED, a red filter would lighten it, but would also turn the yellow (or dulux gold) "refrigerator" lettering white. Such is not the case.

If the top stripe is Blue, a blue filter would lighten it, but would also turn the "refrigerator" lettering dark, which it did  Blue is the only color filter that will turn yellow or gold lettering dark.

Therefore, the top stripe is blue.

Dennis Storzek