Here is the book I am looking for from my childhood. I was born in 1981 for reference.
Book is about a whale and boy. They travel down a river and as they go the whale changes colors from the surrounding areas. I know one page is a jungle with monkeys. The last page of the book is the whale with the boy on top of him swimming out to sea. All the colors have been washed off of him and the water behind him is colored like a rainbow but the whale is white/gray.