You can only change the background color for the Front and back cover combined. (this does not color inside book pages!)
If you select 'Single Page' view, you will see the Front and back cover in total.
Click on the cover to select it, put a tick in 'Background Color', choose the color. (I chose green)
If you don't want the 'Background Color' to appear on the Front cover, use an image that totally 'covers' the front cover.
Inside the book you can color every page a different 'Background Color' if you remove a tick from "Apply Background Globally"
Click in the vertical panel to choose color saturation if you see the grayscale panel first..
Hint: Click and drag the 'dropper' off and away from the color panel and drop it wherever you want to select any color from anywhere on your computer screen!