Mplsjct: As far as I know there were no GN observation cars painted BSB. Business cars, yes, but not passenger cars.
I didn't think of the observations, as they were gone from the Empire Builder before the merger, and, as far as I know, were removed at the beginning of the BSB era, in 1967. The Mountain series, from the 1955 EB, had already been reassigned to the Western Star, and ended up on the Internationals, running until Amtrak. The Coulee series, rebuilt from the 1947 EB, were put on the new EB when the dome-lounges arrived.
I know that the Mountain series cars remained green and orange until Amtrak, as they came through Pasco, one at a time, on the Amtrak Empire Builder, headed east for disposal. What happened to the Coulee series cars I have no idea. As far as I know they weren't used after 1966, and MAY have been rebuilt into something else.