In the Notes app open a new document. There will be a customizable toolbar near the top of the screen where you can select and adjust the S Pen color, width, and several other settings. In the upper right corner, tap the three vertical dots and a menu will pop up with a background color option. The Notes app has another toolbar at the bottom providing settings for type fonts and document formatting. You can even have the app convert S Pen input into text. If you search YouTube for Samsung Notes you will find quite a few tutorial videos about the app.
