Eric Scott 
Day the Rainbow Ended [PDF ebook] 

When the world is drained of all colours David the Snake Dragon comes alive once more and takes Kellie and Gregory on a journey to the Land above the Clouds where the evil gnome known as The Painter has stolen the rainbow and broken it into separate colours so they can’t mix and create real colours. The trio battle their way through the different colour zones in a quest to find true colours so they can repair the damage, but there are many pitfalls and dangers between them and the end of the rainbow. This is the second book that features David, Kellie, and Gregory. The first was The Island of Lost Dolls.