Can I choose a different product option (e.g. fabric color or foil color) after I've started editing my book?

Yes! If you select the hamburger menu (the three lines) in the top left corner of the page. That will open a menu that includes a section titled "change product options." If you click on that option you will see a screen with the options applicable to the book you selected. So, you can switch your foil color, frame color, or fabric color within the editor and preview it as you build!