GoodNotes has various different default background colors for templates. If you're looking to change the background color of the template, then you can do so while viewing the Notebook you're looking to change.
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.