Hi there!
With regards to your concern. You also need to check some settings on your Windows after selecting a "Solid Colour" as your background. Let us start with this:
Open Settings → Personalization → Background
- Under Personalize your background, select Solid color
- Choose the color you want
Then go to: Go to Settings → Accounts → Windows backup
- Turn Remember my preferences OFF, or turn OFF Personalization
Other details to customize your background is also available on the Microsoft support page: https://support.microsoft.com/en-us/windows/change-the-desktop-background-in-windows-4a33c533-b71e-447e-b021-a27df2678938?utm_source=chatgpt.com
I hope this helps somehow but feel free to post back if you have other questions and we fellow members will do our best to help.