Looks like it is possible to use a HTML code block with a Squarespace personal plan website, so we could go with neutral "plays well with others" colors overall and customize the H1 headings -- or some text -- for each book page.

The code block below shows different color embedded into the current H1 heading.​ (It's an arfy color combo . . . for demo purposes only!!

The site background is a different animal which requires CSS coding only available at business plan hosting level or higher.