I do not know about the Spokane International's former GN caboose making it into UP paint. The source that I have is Clive Carter's book: "The Spokane International Railway." On Page 148, there is a photo of a GN caboose that has been painted in SI paint. The caption reads, "Upgrading of the caboose fleet began in 1950 with the purchase of three secondhand 25' cars on steel underframes from the Great Northern." Due to the guessed age of the caboose when purchased by the SI, I would doubt if it made it into UP paint, but weirder things have happened.
Tom Todd