The 1939 Truck Data Book makes no reference to exterior paint (standard or optional) that I can find.
There are however, several depictions/drawings/photos? of various sized trucks where the fender color was the same as the body/hood color. There are no depictions of a truck model that had fender that appeared to be a different shade from the body shade (these are gray-scale "colors" so a red or dark-green truck with black fenders would not be apparent from viewing this book). These depictions would be of "standard" exterior paint.
Try contacting Keith Salter with your question. As you probably know, Brewster Green was the standard exterior color in 1939.
Or, contact Whitney Haist (bottom of the above page-link). It looks like his truck was originally all red but it sure looks nice with black fenders.
