The colorful patterns on the inside covers of my books are called end sheets or end papers. An end sheet is like a theatre curtain in that it appears at the beginning, before the play starts, and at the end, after the play has finished. I pay great attention to the design of my end sheets; they often reflect something about the book and set the tone for the story.
I am especially fond of the end papers in The Very Hungry Caterpillar, pieces of colorful paper with holes in them, because they remind me of how the book started: from punching holes with a hole puncher.
For me, designing the end sheets for each book is part of the challenge and the pleasure of creating a book.
Here are some examples:
From Head to Toe
Papa, Please Get the Moon for Me
1,2,3 to the Zoo
Baby Bear, Baby Bear, What Do You See?