Tom, looking at that page in the book, he says that was the standard for September, 1918. I'm not going to dispute that at all.
Nothing, however, is said about the San Juan/Shavano equipment which was rebiuilt in 1937. You will notice on the same page that all "iron" in 1950 and 1955 is black. That's how it was in 1937, too. There are no pictures in #25 that would dispute this.
But I learned a long time ago never say never.
By the way, the "black" ends he refers to on the ends of RPO's and Baggage cars isn't really paint, it's more of a thick fire retardent goop.
This book is a great resource for anyone with an interest is D&RGW Passenger stuff. Except for B-7, and that's not his fault. All the folios are wrong.