for Gus and Button by Saxton Freymann and Joost Elffers
From the Publisher
Living in a bland, monochromatic mushroom land, Gus and his dog, Button, are surprised when something bright green lands nearby. Gus is determined to find the source of this green thing, and he finds it in a land awash with brilliant color. But it turns out that the green thing is not a thing at all! Full-color illustrations.
Publisher description retrieved from Google Books.