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.
I have been looking for this book as well and think I have found it; it is maybe a polar bear that rides the whale instead of a boy…
The Animals Who Changed Their Colors
by Pascale Allamand
English version by Elizabeth Watson Taylor
1979
http://Www.kathleendeady.com/TheAnimalsWhoChangedTheirColors.htm
Yes! This is it! I’m so excited you replied as I have been talking about this book with my mom for about 5 years. Totally makes sense that I thought a polar bear was a boy.
You made my day!