GoodNotes ships with different default background colors for templates.
To change the background color of the current paper:
1. Open that page, tap the More (...) icon in the top-right corner > Change Template.
2. Tap the Paper Color option to set the desired color (White, Yellow, Dark).
3. Tap a template.
4. Tap Apply.
Unfortunately, it's currently not possible to change the background color to any color other than the default ones. But you can always create a custom template in a desired color and add the template to Notebook Templates.