If you're planning a model, I'll note that the sides of the pendulum cars had a slight curve--they weren't flat.
I'm looking at a photo of the Santa Fe car "just after delivery", and it appears to possibly have a silvery roof, but the sides are painted in three colors. Since it's a Santa Fe photo, it probably shows up in the "Warbonnet" article. The photo also shows up in a two page spread (p. 138-139) on the pendulum cars in the book "The Streamline Era" by Robert C. Reed. There is also an end view of a loose truck in the article.
There's a color picture of GN's pendulum car (GN 999) in its modified form on page 20 of "GN Color Guide...." by David H. Hickox.
I'll note that GN 999 in its original form was one of the coaches assigned to the Portland section of the Empire Builder in the "early days"--say around 1950 or so.
The cars (including the modified GN one) were produced in (unpainted) brass.
Ed