I don't know it for a fact, but I've often suspected that Jack may have colored a lot of those Sunday pages himself. The heavy use of secondary colors and other quirky palette choices always reminded me of his watercolor sketches and self-colored spot illos compared to the more conventional style of coloring that comic book house production artists tended to give him.
You could be onto sump'n. The Sundays were big, bright, and beautiful, but the Sky Cannons wer especially so. I'll post more soon