It depends on the photo book service of your choice, but usually you can change the background, or, to be more precise, its color. However, keep in mind that in some cases, if you change the color of the background, you might end up deleting the theme you have selected on the page you are editing. To avoid this, either choose a theme or a background and apply it to the whole book – unless you want to make things a bit more colorful on purpose.