Notebooks for iPhone Answers

First of all you need to enable "Custom Colors" in Notebooks' settings. Then there you have three options:

1) to change the default for the whole app, open Notebooks' settings and set the color and pattern in "Colors"
2) to change the default for a specific book and its contents, open the book's info page and select "Colors"
3) to change the settings for a note individually, open that note's info page and select "Colors"