Quest by Aaron Becker is the sequel to wordless picture book, Journey. The girl and boy team up again, this time to help an imprisoned king and his kingdom from invasion.
In the previous book there were two colors of markers (which look like chalk to me, but the blurb says marker): red and purple. Now with the king's help, they will unlock the remainder of the rainbow.
As rainbows are really circular, so is this quest. By following the clues, the girl and boy will travel through the rainbow and return where they started, but armed with the tools they need to over throw the invasion force.
It's a beautifully drawn adventure story that parents can use to help children practice their colors.