For the Del Rey edition of The Purple Prince of Oz, Michael Herring provided an image of Kabumpo, Jinnicky and Randy sliding down a rainbow crossing the Deadly Desert into Oz. This incident was written well before the MGM film and hit song Over the Rainbow.
Here we have another wonderful pink sky, and a very apprehensive Kabumpo. As the travelers are heading into the Winkie Country of Oz, where everything is yellow, I'd expect a more golden sky - but the pink is very effective.
John R. Neill also illustrated this scene, from a different angle. I think I prefer his rainbow, as it looks a bit more subtle in terms of color.
The image on the book cover is cropped, as always. In this case, I think it loses the sense of space and traveling through the air. Ruth Plumly Thompson did like to send her characters flying!